bloom out is a regular occurrence, in my garden. i have very few large clumps with a few exceptions. two of the irises that fit into the exceptions category have been 'navy blues' and 'concertina'. ( 'c' is a reblooming intermediate.)
i haven't had 'nb' for very long, but under other conditions than my garden, i think it could become a bully.
my garden for the most part, is dry, sandy, lacking in organic nutrients (tho' i try to provide more), gopher ridden (DH has trapped more than he can count**), and we have alkaline water, with
very little annual rainfall.
i've cut off bloom stalks, fertilized heavily with osmocote; and the results have been loss more than gain. so, the varieties i grow, have to be 'toughies' or they don't stay long.
** we now use a device called a gopher hawk that has been quite effective in this small garden, and less preparation than the other types of spring loaded traps.
https://www.amazon.com/dp/B00KXE6RKK/
while these clumps may not be large for most garden standards, they are for mine.
