Since there are no ridiculous plot contrivances in B7, it couldn't be a ridiculous plot contrivance, therefore Blake must have a good reason for his repeated failures to kill Travis.
Perhaps the question is like that of Hamlet's delay in executing his revenge. A popular current theory is that Hamlet is unwilling to kill Claudius because Claudius has merely succeeded in achieving what Hamlet failed (to kill his father and marry his mother). If anything I'd think it would send him into a jealous rage, but...
So the question is, is Blake suffering such despair that he can't entirely blame Travis for trying to kill him (and keeps refusing to kill Travis to give Travis another shot at him?)
-(Y)