I'd love to find a reasonable solution to this myself, but after talking to someone from Pool Step Specialists I was advised to leave these blisters alone. Ours are even more unsightly than yours - though I have no indication that these are leaking at this point. In the photo below, the dark spots are either where the resin is decaying or cobalt staining from additives in the step manufacturing. The light is not good, so it's not so obvious that these are actually blisters but each of those dark spots is a raised bump in the surface. This pool is approx 23-24 years old, we've only owned it for the last 3 years. I have no idea how well balanced the water was kept before we owned the house, but it seems that these spots have gotten more noticeable since we've been here. I don't know if that's just coincidence and timing given their age, or if it's something I've done that's accelerated the process. We did switch to salt water after the first year we moved here but have no idea if it had any affect on these.
We both researched this independently, trying to come up with ideas to make these look better (the attached photo doesn't show the extent of the blistering - there are quite a few and they are very noticeable) - and have both come to the conclusion that there is no easy solution. We will be replacing our liner this fall, so were really hoping to come up with
something that would make these look better. I believe what we've decided is to use acetone on these black spots when the water is drained, and hope that will help. Very frustrating to think that we'll spend all that money on a new liner, and these steps will still be as unsightly. It's my understanding that the process for removing blisters is both long (including at least a week to allow the blisters to properly dry out) and involved if done properly.
I suppose the other alternative is to replace the steps, which I would assume would cost a pretty penny between finding something that would fit the opening, demo, and redoing the support and concrete.
I certainly don't intend to scare you with what could happen - it's just what we've seen in our almost 25 year old pool. I have no idea how common it is for those blisters to evolve (devolve?) into these spots.
Do keep us posted though - I've had a hard time finding many instances of these types of repairs being done - at least instances where they came back and updated re results and outcomes.