I'm not going to hunt down which fumbles were on what kind of plays, because both are statistically non-receivers. If we changed "fumbles" to "touches", there would be little change, so I'm not going to go there.
Blount had 9 fumbles in his first 2 years, but only 3 in the next 2 seasons. He also has a higher YPC, both as a member of the Patriots and overall. So, for Blount, his fumble rate in years 1 and 2 was one fumble per 42.8 carries. In years 3 and 4, his fumble rate was one fumble per 64.7 carries.
Ridley, on the other hand, has gone in the opposite direction. In year 1, 87 carries and 1 fumble. In year 2, 1 fumble every 72.5 carries. In year 3, 1 fuble every 44.5 carries.
If you isolate Blount's last season, he was still fumbling once every 51 carries, which is better than Ridley.
Breaking things down that way, Blount is trending better than Ridley. The question is whether the trends are real, or just statistical noise.