May I suggest you start a new (separate) thread for this & post pics if possible- one blister can easily turn into more & should be addressed promptly. Skin diseases in snakes are no fun & often the result of excessive humidity + dirty cage (pathogens) that thrive in the warm temperatures provided & needed by BPs. Or it might indicate something more systemic, so please do not ignore this. Snakes often give very subtle symptoms for us to go by. It's good though that your BP has gained some weight- I take it she's been eating?