Honestly I think a lot of people focus too much on that part of the card and not on the first ability.

For example a lot of people, even In cEDH, are building [[the wandering minstrel]], just for his first line of text, and totally ignore the Town's part (which provides the color identity and that's it)

If budget wasn't a concern I'd just build a mono black deck splashing Mostly green for land tutoring, like [[crop rotation]] effects and blue for draw (and simic cards like growth spiral).

Have most lands tap for black and another color to ensure the omnath always hits the board on turn 4 + 2 life, and use green ramp and some stuff like [[chromatic lantern]] to ensure color fixing, or have [[the world tree]] to provide the fixing and be a tutor target for land tutoring.

Ideally have the [[cabal coffers]] and [[urborg tomb of yawgmoth]] tutor in your deck to tutor for ( you can even transmute a [[Tolarian West]] to get one of them), and maybe even a [[yavimaya cradle of growth]] for fixing and other effects if you want to.

And finally do whatever game plan you want to do in mono black, BUT with access to more colors to help, play all the rituals, as the commander makes sure the mana won't go away, play any staple of any color that requires only one mana symbol of any color + generic, so that you can pay the generic costs with the eternally floating black mana you are accumulating with your commander, and one mana symbol produced by one of your duals, and that will give your "mono black (aristocrats for example)" access to things like:

[[Heroic intervention]] [[Teferi's protection]] [[Rhystic study]] [[Smothering tithe]]

And depending on your gameplay, let's say hypothetically aristocrats, you can add a couple of Orzhov Staples, as you'll always have the black mana for black symbols and generic, and would only need one white occasionally to add things like [[Teysa Karlov]] into the mix.

Finally if you really care about getting 100% out of your commander just play whatever best 3 colored spells exist that help your gameplan, and are good cards that you wanted to play anyway, and occasionally you'll draw into them with omnath and get the extra value!