Black Tern
One record:
2004-08-26 - one seen distantly from the Lake Platform
On the 25th, we saw two Common Tern from the Lake Platform,
and the Black Tern may have been there too, but we have no confirmation.
On the 26th, the Black Tern spent most of its time near Idylwood Park (south of Marymoor on the west shoreline).
At times it was identifiable by scope from Marymoor. The two Common Terns were seen there too, at least some of the day.
On the 27th, the Black Tern was again seen from Idylwood Park, with no sign of the Common Terns.
Unknown if the Black Tern could have been seen from Marymoor on the 27th.
I have no photos of the Black Tern, neither from Marymoor nor from Idylwood, but it appeared to be an adult in non-breeding plumage.