The ink sac cannot be used to make black dye, nor can it be combined with white dye to make gray dye.
Note:
The ink sac is traded from the traveling merchant.
I tried another map and this problem didn't appear.
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duplicates 1
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Thank you for your report!
We're actually already tracking this issue at MCPE-101087, so I will resolve and link this ticket as a duplicate.
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Just had the same issue. Purchased ink sacs from a traveling trader and can do almost nothing with them except dye beds, shulkers, wool, the fireworks charge, etc. It's like it says it's an ink sac but is acting as dye for only some things.