Thanks all for advises.
I’ve studied the internet I the meantime, and found only entries for runners’ blisters (on the feet). And just as your recommendation vary there are two ‘schools’, one advocating and the other not advocating to kill the blister.
I killed the first and last one I had, and I found the unprotected raw flesh pretty painful. This time I opted to leave it alone. As I found in one suggestion for the treatment of foot blisters, I am covering my gland it with a little bit of cream an - as a natural plaster - with my foreskin which I keep in place at night using a hair tie and a wrist wrap, during daytime using some pe-weights.
Although my gf was pretty disgusted at first sight we had sex (only mild discomfort while not thrusting too heavily) and the blister didn’t open.
I’ll keep you posted.
Regarding the other technique I read that one can use a sterile needle and a sterile yarn and pass it through the blister; leave the yarn in the blister so that half an inch looks out at both ends. This should allow the blister to be drain it’s fluid constantly. I’ll try this if I should be so unlucky to get another blister some day.