Vowel shift in US English?

[Molly]

Is it true that many people in Northern US cities are now starting to pronounce the word "busses" as "bosses" and "block" (as in street) as "black"?

[Yankee]

.
What do you mean by "northern"?

If you want to know whether I've heard people using those pronunciations, the answer is no.

If you want to know whether I think those "new" pronunciations are becoming common in my neck of the woods, the answer is also (obviously) no.

If you want to know whether it's possible that those alternate pronunciations are currently being used somewhere, I'd just say, sure, anything is theoretically possible.

(It sounds to me as though you've read a little something about vowel shifts in the US, but haven't actually read very much. But that's just my personal impression of your question.)
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[Molly]

[Yankee]

[Molly]