You seem to be thinking that nobody considered black holes as dark matter candidates, but that's not the case. The two big theories around the origin of dark matter are called WIMPs (weakly interacting massive particles) and MACHOs (massive compact halo objects), and MACHOs include black holes. The reason MACHOs aren't widely accepted is because the evidence (e.g. lensing measurements) doesn't suggest there are nearly enough massive invisible objects to account for a significant portion of the dark matter.
I'd kinda worked out that people who know about this stuff didn't think it was a good account. Was more remarking that calling things dark and black (and, indeed, wimps and machos) sort of invites simplistic and erroneous pop-science assumptions.
