Nothing Blenner does is inherently of a capital nature. He hasn't broken and run in combat, or allowed his men to; he hasn't spilled any particularly important secrets or gone around spouting blasphemy. He hasn't been in front of a firing line because he hasn't done anything to deserve it. (You might well say this is because he hasn't been given much chance to.)
So why hasn't he been punished for lesser offenses, such as lacking martial zeal? Well, from the perspective of the Commissariat, he has been. When we meet him in First and Only and again in Blood Pact, he's clearly been sidelined and dumped on a garrison unit (or a succession of them, I don't recall which) that isn't expected to see much if any combat in earnest. For someone like Gaunt or Pius Kowle, the commissar from Necropolis, this would be a serious blow both personally and to their career. In fact, Kowle's assignment was a punishment engineered by Gaunt for his overzealous approach.
And the fact of the matter is that units like the ones he's been assigned to need commissars, too. Their duties aren't about leading men into the line of fire, but about the realities of maintaining law and order over hundreds or thousands of armed soldiers who are not, in fact, doing anything for long stretches of time. Consider Ciaphas Cain, whose nominal duties have little to do with heroics and a lot to do with filing stultifyingly boring paperwork, clearing petty offenses with the local constables, and handling disciplinary issues for the troops under his stewardship.
So from the perspective of the Commissariat, they've solved the problem and simultaneously put the right man in the right position. Blenner may not be the ideal frontline commissar, but he's right at home overseeing a garrison regiment, and this prevents them from having to assign a Gaunt-style firebrand there who would be unsatisfied with such a role.
As for why Blenner was assigned to be a Commissar cadet in the first place, well, imagine how much worse he'd be if he was an Arbites or in the clergy. At least here there's a limit to how much he can abuse his position for luxury.