The Greenway in Davidson is a place I have visited several times a year over the past three decades. It usually has a singing Indigo Bunting in the warmer months, but this was the first time I encountered a Blue Grosbeak there. The two species are both small blue finches with conical bills and warbling songs. Both species have only one song-type per male, so variety is not helpful in distinguishing them. Despite those similarities they are easy enough to tell apart visually and aurally.
The grosbeak is larger with a proportionally larger beak. Its song is lower, with less bandwidth (vertical range on the sonogram). That latter feature makes it sound more finch-like and less warbler-like. To me, the Blue Grosbeak sounds a lot like a Purple Finch. And if you listen carefully, you will notice that the Indigo Bunting repeats each note, while the Blue Grosbeak doesn't. Both are pleasant to listen to, and they sing incessantly in early summer.
Why is today's species called a "grosbeak" when all the other species of Passerina are called "buntings?" Both terms are catchalls, describing a body-type that is found in several bird families. "Grosbeak" is applied to birds with especially deep beaks, for cracking large seeds. "Buntings" are more gracile, but all of them also have conical beaks, also for cracking seeds, albeit smaller ones. It's irrelevant in summer, when they eat mostly insects. The Blue Grosbeak is North America's smallest "grosbeak," but it is different enough in appearance from the Indigo Bunting and the other colorful "buntings" of North America (Genus Passerina) to have been described in a different genus, Guiraca. Genetic studies showed that it belonged with the buntings, though, and so it changed its name. How it became bigger than the others remains a scientific mystery.