Reading first at the documentation of difflib, I thought that the use of junks would have produced the result 

s = SequenceMatcher(lambda x : x == " ", "abcd efgh", "abcdefgh")
s.get_matching_blocks()
>>> [Match(a=0, b=0, size=8)]

At a second lecture, it is clear that such evaluation will return in fact two matches of length 4.

Would it be nicer to have get_matching_block return the length 8 match ? 

Don't know if it's in the spirit of the lib, I'm just asking.

Currently return tuple (i, j, n), means that a[i:i+n] == b[j:j+n], where both matching blocks are the same length.
https://docs.python.org/3/library/difflib.html#difflib.SequenceMatcher.get_matching_blocks

This would not be the case if a has an ignored space and b does not. Changing the current definition would break existing code and would require quadruples to return two different lengths.  This would require either a new parameter for the function to select the behavior or a new function with a new name.

Either option would require justification by actual use cases.  I cannot see what they might be.  An way to have junk chars completely ignored is to strip them from both strings before calling SequenceMatcher.