Just noticed a dark spot on the shaft of my penis. Wasn't there a couple of days ago. What could this be?

Is it a brown pigmentation or reddish or bluish under the skin? The latter would be a broken blood vessel, a bruise, usually no big deal, since there are so many.
Pigment might be anything from a freckle to a mole to a herpes spot to any number of harmless clumps of melanin, the brown pigment that they are made of. Both men and women are prone to all sorts of such spots because of the closed in and moist area in the crotch. If you are worried, see a doctor, and research pictures of skin conditions.