Bear in mind he only had 12 3rd down carries last season in total. I don't think you could make any meaningful conclusion from that. But FWIW, he had 8 3rd and 1 carries. He picked up a yard or more on 5 of those 8 carries. You could mix in a 3rd and 3 that he didn't convert if you want to cover short yardage and put him at 5 of 9. I don't have defensive data for those snaps, but can assume they were heavily loaded up for the run but he still converted more than half of those chances. Meanwhile the great Adrian Peterson converted 5 of 6 3rd and 1s, but extended out to 3rd down and 3 or less, only 7 of 10 conversions. Again, small sample sizes here.
As for yards and YPC, yes, the Colts game padded both YPC and yards. But the Redskins game, he only averaged 4.4 YPC. He had a higher YPC against the Texans (5.3) and Cowboys (5.7) than the Redskins game. And he only played in 11 games, but in 7 he averaged more than 4 YPC. In 2, he didn't get 10 carries (Jets game had 3 carries as an example). He had 50+ yards in 9 of those 11 games despite only getting more than 19 carries once (Washington, 29 carries for 129 yards). He was fairly consistent with his production throughout the season.
Again, not nominating Blount for the Patriots HOF or anything. But I think the memory is often tricked into reaching strange conclusions, and Blount gets the brunt of unfair accusations lately.