Essentially every piece of mathematics research that is later published is first put on the arxiv in pre-print form. Most mathematicians subscribe to the daily arxiv email updates for their subfield and will take notice of any paper close to their work – and the identities of the authors. It is exactly these closely related works that mathematicians are usually asked to referee (precisely in virtue of their expertise). So I would guess that the blinding is not very effective.
[0] https://escholarship.org/uc/combinatorial_theory/authorGuide...