Does this means that blues were only in Mississippi Delta and Chicago or that blues was everywhere else except those two places?
It doesn't mean either of those things. This
probably means that blues music wasn't very important to most of the people who lived outside of those areas. Blues music was far more important to people in the Delta region and Chicago than it was to people in other parts of the world.
To say that blues music "was marginalized" seems a little odd to me. I'm not at all sure that anybody was trying to make blues music less important than what it was. I suspect the author chose "was marginalized" to say that blues music wasn't as important to people outside of those two regions.
Please remember to give us the name of the author and the title of the source when you quote something, Amu. Thank you.
Cross-posted with JJ.