What is this small unconnected solder blob on this PCB?

It's most likely a type of fiducial marker, used by a pick-and-place machine to correctly place the SMD parts on the copper side of the board.

That is definitely a type of fiducial marker by the existence of the solder mask clearance. A solder blob forms on the exposed copper when the board is put through the wave solder (by this stage is has served its purpose of a reference point during etching or placement). Normally there are 2 at least 2 markers to allow for accurate X and Y placement.

Its a fiducial. Seems like the designer forgot to remove solder paste marking of the fiducial in the Gerber files.