What Did Bob Know, and When Did He Know It?

Yes, the new Rugosa Coven story is still coming along. Alas, it won't still be called "Bob and the Black Head of Atho" when it's done. Atho's more than a McGuffin, but it would be misleading to put him in the title.

I have this exposition quandary. There are some things I read too much about when I was working on the dissertation, and now I can't tell anymore where the line is between common knowledge and esoterica. And all exposition problems are that much more embarrassing when your protagonist actually is named Bob. So, two questions:

Have you ever heard of Madame Blavatsky?

If you've heard of her, do you remember any of what you heard?