Glad you were there, Joanne! I put a quick report up at http://birdaz.com/blog
We spent most of our time after you left watching the scoter flock; it was about 100:1 Surf:White-winged, about 1,000:1 Surf:Black. We also had even closer views of the apparent Bufflehead x Common Goldeneye hybrid, and a single American Goldfinch was an addition to the morning's slender passerine list.
I did look up some of the differences between female-plumaged Black and Common Scoters. It's subtle, and I'll have to spend more time looking at Common Scoter next time I'm in the appropriate hemisphere. But these appear to be the major distinctions:
Neck: thicker in Black, thinner in Common.
Tail: shorter in Black, longer in Common.
Bill nail: strongly hooked in Black, less strongly hooked in Common; nail often reaches below lower mandible in Black, not so in Common.
Bill: often slightly shorter in Black, often slightly longer in Common; base more swollen in Black, less swollen in Common; upper mandible rises more steeply from nail in Black, less steeply in Common.
Crown and neck plumage: dark extends more broadly onto nape in Black, less broadly in Common; "ending more or less angular" in Black, "more pointed" in Common [this is inscrutable to me].
Head shape: squarer, with steeper forehead and flatter crown, in Black; rounded in Common.
Feathering above gape: "sharply angled (right-angled or sharper) and feathering adjoining upper mandible concave ending on side of upper mandible (latter variable)" in Black; "angle of bill feathering above gape more rounded and feathering adjoining bill normally convex along side of upper mandible" in Common.
Cheeks: often cleaner in Black, often with vertical dark smudge in Common.
These features are from van Duivendijk, Advanced ID Guide (which is why some of the language is a bit odd). Pyle says that the only known characters to distinguish female-plumaged birds of the two species are bill length and shape; unfortunately, the illustration of those characters (p. 139D-E) shows only males.