This has got nothing to do with being open-minded. This is about the Silver Covenant specifically and concepts in regards to it, as the thread implies. You can conceptualize anything, as long as it makes sense. Black high elves of the Silver Covenant make absolutely no sense. This is like talking about quadripedal centaur humans from Stormwind loyal to Lord Hoofmane; very open-minded, yet completely non-sensical.
The Silver Covenant are the same high elves as the ones that all high elves of Quel'thalas were before the Scourge ravaged the kingdom. The only difference is that these live in Dalaran, not Silvermoon. As such, these are pale-skinned, arcane-based elves that have the occasional paladin and warlock. Anything beyond this is no longer the Silver Covenant.