ok so i bought my blaster and had to do a head re-build and when i took off the head i saw that the tops two fins on the back are broke off idk how but is this gonna hurt anything
How small is it?
If it's like a small chip or small amount gone, then no you shouldn't have to worry about it. Even less if they're in the back like you said.
Since the air goes through the front and the other fins are intact, it will still cool fine.
just next time you do a rebuild, make sure the cylinder isnt warped right around where the fins are missing, you really want it to cool evenly, so if you have hot spots it could be bad