I'm not 100% sure, but this could mean the paint was old and falling off. Sometimes bubbles get under the paint and it looks like a blister. What's really weird in this sentence is "distained." I didn't even know that was a word and I had to look it up in the dictionary. The dictionary says this word is "archaic" -- this means we don't say it anymore. Instead, use "stained." I guess your sentence comes from a very old book!

It means that the paint on the door was old and looked like raised bubbles.