That's an interesting factoid, demonstrates that he's better than he gets credit for. But I doubt there's anyone outside of Blount's immediate family who would take him--either career-long or in his prime--over Curtis Martin. In general, comparing YPC as an absolute measure of quality across eras is tricky, because it will lead you to weird conclusions like, just to pick one example, Alfred Morris > Walter Payton.
I agree that the point you're responding to isn't the real issue, and that the stats don't bear it out. Blount having a better career YPC has more to do with their respective eras and offenses than Martin's last few years weighing him down. Martin didn't really have a pronounced YPC downswing at the end of his career - he was averaging 4.6 as late as his second-to-last season. Part of his claim to fame is that he was able to somehow approach 15,000 career touches without any significant decline until his final season.
Having said that, let's be clear here: Curtis Martin was a much better running back than LeGarrette Blount. That's not a knock on Blount, but Martin was just on another level, in large part but not entirely because of his versatility and durability. In 3 years with the Pats--before his prime, no less--he had just under 4,700 yards from scrimmage. It was a different game back then: offenses ran to establish the pass, which meant more rushes and a lower YPC. If Martin played today, he'd average 4.5 YPC no problem.
