He can, for instance, manufacture evidence so as to mislead me, and if he is sufficiently clever, I'll be misled. But I am also aware that my belief state about sorcerers is not as reliable because of possible tampering.

Seems you can calculate P(evidence | Dark) by taking Dark's tampering into account (basically he'll try to get that value as close as possible to P(evidence | no Dark) ), and update based on that. Your belief may not be reliable in that you may still be wrong, but it still already takes all the information you have (i.e. P(evidence | Dark) ) into account.