See it now? No? Ok, how 'bout here?
It's subtle, but it's there. The two dyed skeins (shown here with their undyed sibling) were mordanted together, and dyed together for 75 hours, give or take an hour.
The difference? The one on top (first picture) and bottom (second picture) went back into the dye bath for another eight hours -- after I added a whack (Piggy is not a born dyer, I'm far too loosey-goosey with measurements) of white vinegar to the pot to alter the dye's pH level.
The results were not as dramatic as I'd hoped. So in a last-ditch effort, I threw in some vinegar into a sinkful of water as an after-bath. No change.
What I'd wanted was two distinct shades I could use in colourwork. What I've got now are two skeins so similar to make colourwork pointless, but just different enough to make knitting a single-colour project with both inadvisable. Unless, perhaps, something knit with a strand of each held together.
Here are all four dyed skeins, back together again:
Overall, I'm pleased with the results on the non-superwash wool. But I do prefer the greater absorption of dye on the superwash Kroy Socks.
I'll definitely try dyeing with black beans again. But not for a while: I have a freezer full of black bean soup to get through first!