I have seen this too (darker triangle area) and my guess is that you are seeing through the very thin wood layer to another layer of material below. It probably wraps around the rails for added strength / ding resistance.
Maybe someone from Starboard will give us the real story on this.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.