I think I have solved my blister issue and I am going to share it here in case it is of use to someone else.
I have extremely flat feet and wear prescription orthotics in every pair of shoes I wear, including my roller skates. When I first began to skate, I took the foot insert from my skate out and replaced it with my orthotics (which I had to keep switching between my skates and my everyday shoes because I only have the one pair). I wore two pairs of socks -- one very thin and tight pair, and over that a thicker pair (like soccer socks). I wanted to avoid blisters, and my experience with hiking told me that this was the way to go. The problem is that my feet slipped around in the skate a lot because of the thin socks and also because the top of my orthotic is pretty smooth. When I put a bandaid on my foot, even when I wrapped my entire foot in tape, it would not stay on for more than a few minutes because my foot was so sweaty and sliding around so much. I would get this ridiculous blister during every single practice which would be just about healed by the next practice, whereupon I would get another one in exactly the same place (my right instep).
This is what I do now: I only wear one pair of socks, a fairly thin pair that is tight on my foot. I no longer wear my orthotics. Instead I replaced them with a pair of insoles with very strong arch supports (I use Dr. Scholl's® Adjustable Arch Pain Relief Orthotics) covered by another set of insoles with a very high edge (meaning that instead of being flat the insole is contoured) on the instep (I took these from an old pair of my hiking boots). My foot doesn't slide around so much and the high instep protects my foot from a seam that might irritate it. So far I have had no problems with my flat feet or my blisters. Yay.
Some other skate advice: I have found that leaving the shoelaces fairly loose is a good thing. It doesn't seem like it would be but it is. If your feet are hurting in a weird sore muscle kind of way -- try that. Also, loosening the trucks a bit has helped a lot with my comfort level, as well as my speed.