Who is the oldest Blessed?
Before answering this question, I would like to explain a few questions of nuance, so that one can understand where I am going with this.
First of all what is a Blessed?
The Difference Between a Blessed and a Saint
If you ever had a chance to look at the Order of Preacher's Liturgical Calendar, you'd see a lot of people with the title Blessed. You most probably have never heard of any of them. They're certainly not on the Church's calendar of saints.
Why? That's the difference between "blessed" and "saint." A "blessed" is for a local community; a "saint" is for the universal church. In the explanation of the process, Sanctorum Mater, you'd read that a "blessed" has to have an approved miracle, and is honored in his cult, or community.
Thus the cult of a blessed may only be celebrated in the diocese or country where a particular blessed lived or a particular Religious Order that the particular blessed was attached to. Thus Dominican blesseds can only be celebrated by members of the Dominican Order. Visiting priests who are not Dominicans can not celebrate a Mass in honour of a Dominican Blessed when visiting a Dominican convent, unless they are Third Order Dominicans. Sometimes such blesseds are called saints, but in reality are blesseds.
A modern example of this is St. Hildegard of Bingen was listed in Benedictine liturgical calendars as a saint, but in reality was a blessed and thus limited to the Benedictine Order.
Although the history of her formal canonization is complicated, branches of the Roman Catholic Church have recognized her as a saint for centuries. On 10 May 2012, Pope Benedict XVI extended the liturgical cult of St. Hildegard to the entire Catholic Church in a process known as "equivalent canonization". On 7 October 2012, he named her a Doctor of the Church, in recognition of "her holiness of life and the originality of her teaching." - St. Hildegardis of Bingen (Wikipedia)
One last point I would like to make here is that in the Early Church, canonization was acclaimed by the faithful as the now famous phrase goes: Vox Populi Vox Dei
With all this said, I will now proceed to give three examples of blesseds, who remained blessed the longest.
St. John Cassian (360-435) was considered a saint by some. His cult was never universally permitted. However, Rome permitted his cult in the diocese of Marseilles the city in which he died.
At Marseilles his feast is celebrated, with an octave, 23 July, and his name is found among the saints of the Greek Calendar. - Catholic Encyclopedia
Having his cult reserved to the Diocese of Marseilles made John Cassian, in fact, a blessed and not a saint. However the story does not end here. In the 2001 Roman Martyrology, the name of John Cassian was inscribed in the General Liturgical Calendar of the Church with the title of saint. Thus this holy blessed is now permitted to be honoured as a saint since 2001!
He is a saint of the Eastern Orthodox Churches, with a feast day on 29 February, a date assigned also in the liturgical calendar of the Episcopal Church (USA). Because this day occurs only once every four years on leap years, official church calendars often transfer his feast to another date (usually 28 February).
The Roman Catholic Church also ranks him as a saint, with a feast day on 23 July. Like his contemporaries Augustine of Hippo and John Chrysostom, he was never formally canonized, a process that came into use several centuries after his death. Pope Urban V referred to him as sanctus (a saint) and he was included in the Gallican Martyrology He is included also in the Roman Martyrology with a feast-day on 23 July. Like the great majority of recognized saints of the church, he is not one of the saints in the General Roman Calendar, but the Archdiocese of Marseilles and some monastic orders celebrate his memorial on his feast day. - John Cassian (Wikipedia)
Number two on my list is the famous Catholic philosopher Boethius is considered a saint by some. But again, his cult was limited to the Diocese of Pavia. Anicius Manlius Severinus Boethius’ (480-525) best-known work is the "Consolations of Philosophy" written during his imprisonment — "by far the most interesting example of prison literature the world has ever seen." It is a dialogue between Philosophy and Boethius, in which the Queen of Sciences strives to console the fallen statesman.
There can be no reasonable doubt, then, that Boethius died a Christian, though it is not easy to show from documentary sources that he died a martyr for the Catholic Faith. The absence of documentary evidence does not, however, prevent us from giving due value to the constant tradition on this point. The local cult of Boethius at Pavia was sanctioned when, in 1883, the Sacred Congregation of Rites confirmed the custom prevailing in that diocese of honouring St. Severinus Boethius, on the 23rd of October. Catholic Encyclopedia
Once again, even though called by some a saint, his cult is only liturgically allowed to be celebrated in the Diocese of Pavia. That again means he is only a blessed and not a saint.
The 2001 Roman Martyrology raise Blessed Boethius to the rank of a saint by permitting his cult to be celebrated by the Universal Catholic Church and his name was also inscribed in the General Liturgical Calendar of the Church.
All this said and done is fine and in case his is a little confusing, please allow me to offer one last example: Blessed Charlemagne.
Charlemagne or Charles the Great (2 April 742 – 28 January 814), numbered Charles I, was king of the Franks from 768, king of the Lombards from 774, and emperor of the Romans from 800. During the Early Middle Ages, he united the majority of western and central Europe. He was the first recognised emperor to rule from western Europe since the fall of the Western Roman Empire three centuries earlier. The expanded Frankish state that Charlemagne founded is called the Carolingian Empire. He was later canonized by Antipope Paschal III.
Charlemagne was revered as a saint in the Holy Roman Empire and some other locations after the twelfth century. The Apostolic See did not recognise his invalid canonisation by Antipope Paschal III, done to gain the favour of Frederick Barbarossa in 1165. The Apostolic See annulled all of Paschal's ordinances at the Third Lateran Council in 1179. He is not enumerated among the 28 saints named "Charles" in the Roman Martyrology. His beatification has been acknowledged as cultus confirmed and is celebrated on 28 January. - Charlemagne (Wikipedia)
The liturgical cult of Blessed Charlemagne is permitted in certain dioceses in Germany only. He is still a blessed and not a saint as of today!
After all is said and done, please take your pick.
St. John Cassian was a blessed for 1566 years.
St. Boethius was a blessed for 1476 years.
Bl. Charlemagne has been a blessed for 1205 years and counting.
The book in Latin only Index Causarum Sanctorum ac Beatorum would actually answer every single sub question you mention here. It would also eliminate any doubts in any answers you may receive. It it definitive on the subject of saints, blesseds, venerables and servants of God. However it is extremely hard to come by as it is usually only found in very specific private Catholic libraries (mostly