This one keys out to Elachertus using Burks' online key (1). I can add the couplet #s and character states to this post if needed. The wasp is "black" until illuminated under a hemispherical diffuser. According to Shauff's paper, the "Color varies from flat black with little evidence of any metallic sheen to distinctly metallic green."

Using Schauff's key to this genus (2), this wasp keys out to Elachertus argissa as follows:
couplet 1) anterior margin of scutellum & axillae forming a straight line; dorsellum having lateral lobes and being excavated ventrally
couplet 2) the malar suture is straight (vs. curved in E. atus)

Couplet 2 also uses the posterior ocellar length/ocell-ocular length ratio to distinguish between females of these two species (2 in E argissa vs. 1.5 in Elachertus argissa). I get 1.7-1.8 (under the assumption that Schauff's posterior ocellar length = Whitfield's "inter ocellar distance"). For this wasp, that measurement is obviously not much help...
TIA

larger image here