I know this sounds slightly gross, but my harp teacher told me that if I ever get blisters, to pop it, then dip the blister in methylated spirits, and leave it to dry for a few minutes. This is supposed to harden the blister. Then, you get a nail file, and file back the hard skin to get a nicer tone.
I am lucky in that no matter how much I practice, my fingers don’t blister, so I’ve never actually tried this method myself. I use the nail file on the calluses that form, though, and that works well.