Blocking with leads to a potential seki. Black still has to win the ko at b for this to be seki.
Gronk: White can sacrifice the for life, no? The order of
and
may be interchanged. Still, the correct solution is better.
Kea?: I think that would lose points. The sacrifice gives 8 points to Black and only gains 2 points of territory. The seki is more desirable (and of course the correct solution is even more).
Herman: Note that in the first diagram was a mistake by Black. Black kan make ko for seki like this...