Black ducks?

While I was searching the lake for the loons, this pair of birds came into view. They have white faces on long black necks perched on dark gray bodies. Using the bird books I have, I can't figure out what they are. I'll keep looking.
The loons were elusive this morning but at last I sighted them, bobbing around in the water. It's been a short stay at the lake for us between trips to New York and California.