He was never a firm believer of seers or the future they could see, but this one seer at his magic tower—she told things—things that came mostly true later on to him; it made Blaise trust her and he trusted her even now. He believed with Bree changing, the future the two shared would get better—but the seer told him that it wasn't possible.
Bree was just as much of a disaster as she was in the beginning to him.
She even told him that he would regret claiming her. Thus Blaise hesitated again and again, wondering if it was the right thing to claim her. What if Bree did some great injustice to him, which led to the bond breaking?
He had seen how terrible the effects of a bond breaking could be—when his elder brother's mate walked away from him without even giving him a chance to make things better, his elder brother had drowned in pain, emotions, and nausea, with exhaustion weighing down on him.