Size estimated. Taking nectar on Hoary Mountain-mint, Pycnanthemum incanum, or a similar species. This looks very similar to a wasp identified by Eric Eaton as Isodontia philadelphica, compare:

(Eric verifies this--see comments.)
This is a pretty common species in the North Carolina State University Entomology Collection. I know species identification in this genus is tough. This one, with its blue-black wings, does look different than the ones I usually see, which have more brownish wings:

Those are likely I. mexicana and/or I. apicalis, based on Eric Eaton's identifications, and I. mexicana is common in the NCSU collection, linked above.
Image updated 2019 and additional detail images added.