I'm using a blissey in My diamond team and Out of battle hidden Power is classified as a normal move, so do I get STAB from it?
No. Even though it looks like a normal type move, when you use hidden power in battle it's always one of the other 16 types. You will get stab if the user's hidden power type is one of its types, but not if the Pokemon is pure normal like blissey.
Check out this website: https://bulbapedia.bulbagarden.net/wiki/Hidden_Power_(move)