The butterfly above is an azure, one of several species in the genus Celastrina. Azures used to be easy; if you were in the eastern U.S. in the spring, you would most likely be looking at a Spring Azure. Over the last decade or two, research has shown that, as with many bird species, "Spring Azure" is actually a complex of related species that are similar in appearance but differ by their larval food plants and flight periods.
This means that azures are now difficult to identify, especially in May when multiple azure species could be flying in New Jersey during any given week. It is a bit easier if you can tie an individual to a larval food plant. The azure above and below appears to be ovipositing on a Gray Dogwood (Cornus racemosa), which is a known food plant for Summer Azure (Celastrina neglecta). The other azure likely to be flying at this time is Cherry Gall Azure (Celastrina serotina), which looks fairly similar to Summer Azure, but whose larvae feed mainly on galls on cherry trees (Prunus sp.), though they will occasionally use other food plants. So I think this is probably a Summer Azure.
One other thing to note here is that azures are notable for feeding on the flowers rather than the leaves of their hosts (with the exception of the Cherry Gall Azure). This azure is giving its larvae a head start by laying its eggs directly on the flower buds.