Attached patch, issue10254a.diff, adds the OP's cases to test_unicodedata and changes the code as I suggested in msg124173 because ISTM that comb >= comb1 matches the pr-29 definition:

"""
D2'. In any character sequence beginning with a starter S, a character C is blocked from S if and only if there is some character B between S and C, and either B is a starter or it has the same or higher combining class as C.
""" http://www.unicode.org/review/pr-29.html

Unfortunately, all tests pass with either comb >= comb1 or comb == comb1, so before I commit, I would like to figure out the test case that would properly exercise this code.