I agree with someone's advice about Detroit and St. Louis. While St. Louis is nice, not a lot to draw tourists, unless you catch a Chuck Berry show at Blueberry Hill (he plays there once a month for like $20).
If you're looking for Robert Johnson, read this book, it's awesome:
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www.amazon.com]
While it's about a lot more than Robert Johnson, besides Bob Dylan, he's the biggest running theme throughout the book, and they go to several of the rumored gravesites. Just a good book in general.