Are brief blasphemous deliberate thoughts a mortal sin according to Catholicism?
To be a mortal sin, these conditions must be satisfied:
Knowledge (either natural or supernatural) of the act's sinfulness (offense to God).
Freely performing the sin.
In your scenario, #2 is satisfied because you say "deliberate". #3 is satisfied because blasphemy is a grave offense against God.
It's unclear if #1 is satisfied, you should confess it to a priest, who can help judge if such a sin is mortal.