G'mornin' Chris,
I imagine that they'll have try-outs about the same time next year. You could follow the link above and cruise around the Seahawk.com site. Or, maybe, write a letter to the following address?

Blue Thunder
Seattle Seahawks
800 Occidental Ave S., Suite 500
Seattle, WA 98134

Since I aren't affiliated with the Seahawks or Blue Thunder, that's about all I can offer.
Break a leg,
BillA