I would never do a black sink anywhere and for sure not in a space where soap scum is always an issue . IMO stainless is still the best material for kitchen or laundry sinks for sure and would look awesome with that counter I like one large one basin under mount sink for both spaces both nice and deep too

In the powder room, white sink like the veining in the countertop. Not sure black will look good with it as the stone doesn't really look black - more dark gray. SS probably a good idea there.

Powder room, white sink.
Laundry room, stainless steel.

White in the powder room.

Under mount stainless in the laundry. Nothing easier for upkeep and permanent good looks.

I love your marble choice. GORGEOUS!!!!

If you were doing a black bowl like this maybe a black sink would work. When I think of black, I think of Vegas Casino bathrooms with over the top drama.

Village Retreat · More Info

But then I saw this one.. And Ohhhh wow maybe a black sink could work.

Hilltop Farmstead · More Info

Although.... check out this below. Beautiful stone and the sink is a perfect white!!!

Glass House with Pool Views · More Info

I don't think I helped you at all but boy did I have fun looking at gorgeous bathrooms with similar stone that you are working with. Can't wait to see your finished powder room. It is going to be stunning.