Whenever an opponent casts a blue or black spell, you may put a +1/+1 counter on Mold Adder.
If an opponent casts a blue or black spell, Mold Adder’s ability triggers and is put on the stack on top of that spell. Mold Adder’s ability will resolve before the spell does.
If an opponent casts a spell that’s both blue and black, Mold Adder’s ability triggers only once.