Tom wrote:
Black & Decker miter sale model 7715 type 3.

I bought this saw at a garage sale. the seller cut a board, said it
needs a blade. But, I can't remove the old blade. The armature of the
motor turns when I try to turn the blade nut. If I "jam" the blade so
it can't turn, the armature still turns. How do I lock the motor to
remove the blade?

In addition to the other good suggestions, slather the nut and shaft with
nut-release juice.