If I put dark chocolate hair dye on blonde bleach hair what colour would it go? Or should I go the hairdressers and get it done so it doesn't go a horrible colour if it does go wrong?

You should Go to the hair dressers Believe Me They Know Better!
Tell Them What Color Do You Want to Have And After That Its Like A Piece Of Cake And Its All Done and they will get you the exact color u want :)

I would recommend going to a salon to get it dyed, depending on the exact blond that your hair is will give you different outcomes and may not be the color you were expecting.