The easiest option is to go with your basic E minor pentatonic scales. When that Bb hits, you can "blues it up" by adding in those Bb notes to the pentatonics.
Another option is to play in G, as above. (E minor, G...same thing for all intents and purposes). Then when the Bb is played, switch to a Bb major scale for kind of a funky tonal shift before going back to G.