In a camera ready paper (has been accepted), how to cite my own paper that is still under blind review?

While something is under review, it's in an indeterminate state and might not survive the review process. Once something is published, all of its critical supporting references need to be available.

Thus, I believe that the correct approach here depends on whether paper B actually depends on paper A, or if it's just a "related work" (e.g., "In Paper A, we solved a different problem from the same family of problems."):

• If Paper B depends on Paper A, then I think you have no choice: put a preprint of Paper A into arXiv (or similar), and hope it doesn't hurt the blind review process---and in most cases it probably won't.

• If citation of Paper A is only a "nice to have" for related work, then I would instead suggest simply dropping the citation, since the related work has not yet come into being. When I had much the same thing happen to me, I ended up following the second route, which was the right thing to do.

Nothing, however, can protect you entirely from the vagaries of the scientific publication process: in my own case, the first version of my Paper A was rejected partly because the reviewers took so long that they ended up complaining that I didn't cite my Paper B in it!