If ir is MASSIVE as you say, painful and in an inconvenient spot i.e on the ball of your foot, then no harm will come if you pop it. Sterilise a needle or razor blade in a hot flame or antiseptic, clean the area with antiseptic e.g Dettol and just prick the blister and squeeze out the fluid. Apply antibiotic cream. If this proceedure is difficult due to implements then leave well alone. The problem with a blister on the sole of the foot, is that it will burst when you walk on it and be very painful and may become infected.