Your best bet would be to contact the conference organizers. Although you could call and speak to the Sales Department at the hotel, I could easily see them declining to discuss any details regarding the contract. Maybe they will, since it's just one small aspect.

Also, check the small print attached to the conference registration. There may have been a clause that states that if you book outside the block, they have the right to "take over" your reservation and place it within their block. I saw that occasionally when I was a Revenue Manager.