Ink sacs cannot be crafted into black dye. Using a crafting table, it still shows that the recipe for black dye requires just one ink sac. However, even with ink sacs in the inventory, the crafting table does not allow the recipe to be crafted, as if materials were still missing. The same thing happens when crafting without a crafting table. Other dyes appear to be working correctly.

Edit: Sorry, this may be a duplicate of MCPE-103760, which was a bug report that included two issues. The first issue reported in MCPE-103760 appears to be resolved, but black dye is still not craftable, as reported here.