I had an occasionally recurring blotch like this...

It turnes out to be grease from the hinge on my easel which ended up on my left little finger whenever I adjusted the margin to a larger size.

Almost drove me crazy finding out how I sometimes got grease on one finger, after figuring out why I sometimes had blotches on 8x10" and 9.5x12" prints, but never 5x7" and smaller...