Can't say I've caught any blackmouth or kings on a fly, but I've caught a bunchon gear in the Straits when we could fish for them.
The low-light trigger for shallow kings is right on the money, for the most part. But fishing out of Port Angeles, Freshwater Bay and Pillar Point has shown me that the kings are not restricted to kelp lines. Early in the morning-- just when you start to see dawn coming up a bit-- you'll find kings up just under the surface. I used to fish for them a lot with 25 pulls of line, a plug-cut herring and a fast troll, and I wasn't sticking close to shore either. When I'd hit it right, it was nothing to have three kings (the limit then) in the fish box before the sun cracked the horizon.
Here are a couple of examples, though, that expand on this shallow behavior a bit-- in one four-day period at Pillar Point when it was extremely foggy, the kings stacked up close to the surface in the middle of the bay, and the "25 pulls" presentation was dynamite. The folks who were fishing on the deck caught zip. When the fog disappeared, the kings dropped down.
The other example occurred four years ago in Port Townsend Bay when a school of springers came in and fed heavily close to the surface for about a week. Forty pulls of line and a plug-cut herring trolled at bucktailing speeds was the ticket. Haven't found them there since.
What all this means in relation to flyfishing for these buggers is something else again. I am going to be trying to do just that this summer-- unfortunately, my best spots are in the closure area. But there are a couple that aren't where I've caught smallish kings heading down Sound fairly shallow. If nothing else, it will be interesting.