They're actually beautiful fish. Hoping we can get some underwater shots or video this year. And they are fun, if frustrating to hook when focused on tuna.
As for "common"? Geez, if guys wanted to catch blues on fly gear it wouldn't take much more than some tuna blood and patience, to have them swarming the boat. Seems like it never takes long for the first to show after getting on a school of tuna. Given how many sharks are out there, it would be interesting, maybe even a bit unnerving to deliberately set up a chum slick and see what shows.
Edit: Might be a fun way to kill some time on one of those days when tuna are taking an extra-long mid-day siesta. Blues showing are as close to a guaranteed thing as there is out there, but always the chance for salmon shark, thresher, and of course mako.