Ink sacs that are purchased from Wandering Traders aren't able to be used in crafting black glass, glass panes, concrete, terracota or wool. They also can't be crafted into black dye, and some items appear in the crafting table as needing ink sacs, although even if you have the items and the ink sacs, it will not let you craft that item. I can verify that this glitch affects both iOS and Xbox One versions.
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Thank you for your report!
We're tracking this issue in MCPE-101087, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki
-- I am a bot. This action was performed automatically! Please report any issues on Discord or Reddit