Steps to reproduce
Place a sign and input some text on it.
Use bonemeal on the sign.
Use cocoa beans on the sign.
Use lapis lazuli on the sign.
Use an ink sac on the sign.
Expected result
Same effect in each of steps (2) - (4) Either you can dye signs with these items, or you cannot.
Observed result
Bonemeal, cocoa beans, and lapis lazuli work can dye a signs, but ink sacs cannot.
Maybe this is caused by the fact that ink sacs can be used to remove the effect of glow ink sacs? It's still confusing because ink sacs work as black dye in other situations. I don't see why they can't work to dye signs black when the sign does not have the glow effect, just like using bonemeal, cocoa beans, or lapis lazuli on a sign that is already dyed its respective color allows you to edit the sign.