Except that the Blacks moving from Mississippi to Chicago were consciously leaving the South, unlike those moving from rural Virginia or Maryland to DC or Baltimore. Which is not much different from leaving rural Alabama for Birmingham. And except that there's additionally a very large minority of clearly Southern people among the Whites of Maryland - not just in the rural parts but in the Baltimore Metro as well.
What people mean when they say Maryland is primarily a Northeastern state now (and no one would have dreamt of suggesting that when you were born) are actually saying that its White culture is primarily that of a Northeastern state now. Which is true but a different statement.
I think you are quite incorrect. The black population in Prince George's county is not southern and represents a large middle - upper middle class black population that is arguably unique in the US. The original black population from the slave era declined in numbers throughout the first half of the 1900s. The black population grew after WWII due to employment by the federal government and came from all over the US. If they came from areas in the South, it was to consciously leave that region, much as you recognize in Chicago. As for Baltimore city, there is some common US urban black culture, but urban life in Balto is more like Philly than Richmond or Charlotte, and that defines its sense of region. Balto is a northeastern city by any measure.
I don't recall what views of MD existed when I was born, but by the time I was in college in the 70's MD was talked about as part of the Mid-Atlantic, not the South. Growth in the DC region driven by internal immigration to work for the federal government had been under way for a generation and its impact on the region was recognized by the 70's. That's not based on white views alone.
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