Could use some clarification of S. nigra, S. nigra ssp. canadensis, & S. canadensis. GoBotany has S. canadensis as a synonym of S. nigra; The Plant List has S. nigra var. canadensis as a synonym of S. canadensis; not in FNA yet; PLANTS has S. canadensis & several others as synonyms of S. nigra ssp. canadensis.

Weakley has S. nigra ssp. canadensis & several others as synonyms of S. canadensis, with this note:
"Bolli (1994) treats this taxon as a
subspecies of a very broadly defined S. nigra. He recognizes 6 subspecies: ssp. nigra in Europe, ssp. palmensis (Link) R. Bolli
in the Canary Islands, ssp. maderensis (Lowe) R. Bolli in Madeira Island, ssp. canadensis in eastern North America, Mexico,
Central America, and the West Indies, ssp. cerulea (Rafinesque) R. Bolli of western North America, and ssp. peruviana (Kunth)
R. Bolli of South America. I prefer to retain these taxa at the species level, particularly as Bolli states "the geographical races, in
the following defined as subspecies, turned out to be the biological units in Sambucus." Bolli further discusses 3 races within
what is here called S. canadensis (his S. nigra ssp. canadensis), one from eastern North America, another from montane Mexico
and Central America, and a third from subtropical se. North America and the West Indies; he considers these geographic races to represent "morphological and perhaps genetical" differences, and that "at present, all races are probably interconnected." This
variation may be worthy of taxonomic recognition at the varietal level, and these "races" have formerly been considered to be
species or varieties. If given varietal recognition, plants of most of our area represent S. canadensis var. canadensis, while
evergreen (or tardily deciduous), bipinnate plants of FL, s. GA, s. AL, s. MS, s. LA, se. TX, and the West Indies represent S.
canadensis var. laciniata A. Gray. The variation is clinal, and bipinnate leaves are seen as far north as coastal NC."

3/31/2018 Update:
iNaturalist authorities:
Calls it S. canadensis:

VASCAN
Weakley 2015

Calls it S. nigra subsp. canadensis:

GoBotany

Listed as present in California by USDA PLANTS, but taxon not recognized in Calflora (only S. nigra subsp. caerulea listed).

Tiebreaker:

The Plant List, which calls it S. canadensis

Not iNat authorities, but the following also call it S. canadensis:
Plants of the World Online
Michigan Flora
Flora of Missouri
Flora of the Chicago Region
Weber/Wittmann Flora of Colorado, per @cwarneke above as I do not have a copy

https://www.inaturalist.org/taxon_changes/31482
final thoughts?

Sounds good, thank you for doing all that background.