Page 271 of the PHB. Now that I look, it has the same language under "area attack" so it's not just for close attacks.
Blinding Barrage with a crossbow can be a bit odd, but it's an edge case that I've never seen come up in actual play. The only viable way to play a crossbow Rogue is to go the "sniping with stealth" route, while Blinding Barrage is an "in your grill" power that appeals mostly to dagger Rogues.
Really, the crossbow in 4E works the way it does because it would be completely hosed compared to other ranged weapons otherwise. If I had a player who used a lot of crossbow multi-attack powers I'd probably just flavor it as him having a sweet "customized" repeater style crossbow. He wouldn't get multiple attack with it outside of powers because no-one in 4th gets multiple attacks without powers.