See https://parks.state.wa.us/DocumentCenter/View/1843/Blake-Island-State-Park for a detailed map – there’s a “4 tents” icon indicating the group camp on the northeast corner.
I stopped there on my way across the sound a couple of weeks ago. The corner of the jetty on the northeast end is very obvious (plus it protects a marina, so I imagine it must be bright at night). There’s individual campsites immediately south of the jetty; south of that, the spot where there’s a break in the driftwood for a pathway is the entrance to the group camp area. Satellite view: https://goo.gl/maps/2TntiBpQgqx
If all goes according to plan, the Team HMS Swiftsure yellow rowing canoe will be there on the beach. I can’t guarantee I’ll get there before you, though!
Contributions to defray the cost appreciated. Virtual tip jar is via Paypal at firstname.lastname@example.org.
Real-life tip jar / bailing bucket will be sitting in my boat 🙂