I'd say your blue orb is ink lifted from another stamp. Probably the two stamps were stuck together and yours retained some of the other stamps image when separated.
If you flip your image you'll see what looks like the top of a number two, very similar to that on the 12¢ denomination stamp from the same pictoral issue.
Image from web
Edit: Actually, now that I see the reversed image on the screen, I can make out the horse, so it's probably picked up from another 2¢ horse and rider stamp. Interesting that it is blue.