Has anyone had any experience buying from / through Blackfish marine?
Any other reputable used-boat brokers in the greater Seattle area that
anyone can recommend?

I'm personally acquainted with Patrick Kelley, the manager at Blackfish. I also
converse regularly with his salespeople.

I like the tone of your question. Any *other* reputable brokers. The guys at
Blackfish will treat you fairly, honestly, and professionally- (as you
apparently expect based on the phrasing of your question).

You could do far worse than deal with Blackfish. It would be difficult to do
much better.

If you're shopping for a new Silverton, Cruisers Inc, etc, you will certainly
be doing business with Blackfish as they are the exculsive dealers for these
boats in the area.

If you're shopping for a used boat, you can actually use almost any broker-
they co-broker inventory. There may some microscopic advantage to going through
the listing broker on a used boat, however.
The listing broker is probably more familiar with the boat, and the greater of
number of ways the sales commission is going to be split up, the less motivated
the listing broker may seem about getting an aggressive offer approved by the
seller.

Blackfish Marine in Seattle: good guys.