I really don't know that "a block" has ever been a predetermined measurement.
I've always thought that it was just a figure of speech.
"To go around the block" always meant to me that you would ride/walk/drive around the streets that were the boundary of a group of houses.
We used to say when we were kids that we'd do a blocky on our bikes which meant a lap around the neighborhood.
Or take the dog for a walk around the block.
Another strange use of the word is "doing your block"
(Losing your temper, spitting the dummy, carrying on like an epileptic rooster)
Never been able to work out what your block is.

Although I have done it!!!