Dumb! I see what it is and if anyone has the same problem this might help. In Settings/Media there is a field to direct uploads to. Mine was set to include the wrong path. Reverting to the WP default for this field solved the problem.
I’m still a bit curious how this setting, which I may have set on my old blog but definitely did not set on the new blog, was retained. The old blog, on another host, was removed. The new blog was not a move of the old files, but a rebuild of WP from the ground up. I did not import anything. Maybe cache?? No point, just curious now.