My first guess was blueberries; maybe I was thinking "high bush." They are, rather, Allegheny serviceberries, one of several serviceberry species found in the five boroughs. Like their more familiar relations, they wear a flared crown opposite the stem and ripen to purplish-blue. Reportedly they're also sweet and juicy, with a blueberry-like flavor; however, the High Line has a "no picking" rule for all its foliage. Allegheny serviceberries do grow wild elsewhere in the city, but probably nowhere in such temptingly easy reach.
The High Line
Near Tenth Ave., between Gansevoort and 20th streets