iNaturalist's authority for vascular plant taxonomy is Plants of the World Online aka POWO, plantsoftheworldonline.org. Like some of iNat's previous regional taxonomic authorities (VASCAN, Weakley), POWO splits Sambucus nigra into several species rather than treating them as infraspecific taxa.

Although Sambucus canadensis (syn. S. nigra subsp. canadensis) has been an active taxon on iNaturalist for a while, there are still ~1300 observations identified as Sambucus nigra in eastern North America that should likely be identified as S. canadensis as well as ~1200 in western North America that should likely be identified as S. cerulea (syn. S. nigra subsp. caerulea). This split serves to illustrate that Sambucus nigra on iNaturalist is in the strict sense, "sensu stricto", a species native to and largely found in Europe, but also introduced and naturalized elsewhere.

The output taxa have been atlased on iNaturalist--rough range maps uploaded. Range maps are never perfect, and this group is complicated by the fact that several species have been introduced and naturalized in many different countries worldwide, so please be vigilant in reviewing observations of Sambucus spp. Unfortunately Sambucus nigra sensu stricto has apparently been documented in Ontario, Connecticut, Virginia, and Pennsylvania, where it co-occurs with S. canadensis, so observations currently identified as S. nigra in those locations will be bumped up to genus rather than automatically reassigned to Sambucus canadensis. There are a few other similar cases of co-occurrence worldwide.

For eastern North America black elderberry ID, adapted from GoBotany and Michigan Flora:

For western North America black elderberry ID, to my knowledge the only dark-fruited elderberry out there is Sambucus racemosa var. melanocarpa (and rarely S. cerulea), so be on the lookout for S. racemosa var. melanocarpa that are now incorrectly IDed as Sambucus cerulea...

Atlases and observation count stats, pre- and post-split [2018-09-27]:

• Sambucus nigra (sensu lato), 4210 obs -> 22 obs (due to users opting out of community ID and taxon change updates)
• Sambucus nigra (sensu stricto), atlas, 0 obs -> 1890 obs
• Sambucus canadensis, atlas, 1489 obs -> 2879 obs
• Sambucus cerulea, atlas, 2746 obs -> 3994 obs
• Sambucus lanceolata, atlas, 0 obs -> 0 obs
• Sambucus palmensis, atlas, 0 obs -> 0 obs
• Sambucus peruviana, atlas, 10 obs -> 14 obs