Are brief blasphemous deliberate thoughts a mortal sin according to Catholicism? Are brief blasphemous deliberate thoughts a mortal sin according to Catholicism?
Mortal sin (from mors = death) forfeits sanctifying grace and is the supernatural death of the soul. St. Augustine defines it as "something said, done or desired contrary to the eternal law."

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.
Grave matter


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.