If you're talking about what Crossfitters deem "butt wink", I know there's a big debate over if it's an issue or not. I'd think that if it's not due to tightness/weakness in the glutes/hamstrings/hip flexors, then it would simply be a function of your bone structure due to either anterior or posterior pelvic tilt, which isn't uncommon.
Like CDB says, foam rolling won't hurt. I've done it on my hamstrings in the past and I definitely increased ROM. It's not good to do as a warm out though so do it either after your workouts or on off days. If you deload, you can throw it into your deload/active recovery week too.
Also learn how to PNF stretch and PNF your hip-flexors (you can PNF your other stuff if you have a partner or resistance band). Needs recovery days in between PNF sessions.
If hamstring flexibility and hip mobility are increased but "butt wink" stays the same even when using no weight, I'd think it's a function of your bone structure.
The block under your heels thing; wouldn't that be placing your foot into planterflexion, which would be similar to squatting on your tip-toes? Unless I'm mistaking the utilization of the block, I'd think that'd be horrible for your knees. Maybe Riptoe was just using it as an example; not necessarily as a "go-out-and-do-this?"