The key word in the phrase is "allow." The other daughters were single and as you note called daughters, and all received a cow, a heifer, a calf, a bed and spinning wheel.
Adam Black stated "I do allow Mary Sights & Catren Mock Eatch of them to have ten pounds." If they were non-relatives or relation other than daughters, the phrase wouldn't be "allow" but would be "I give" or I bequeath" or something similar. That word "allow" implies that they have already received from his estate as heirs, and he was "allowing" them to have another 10 pounds each.
Next, there is proof that Catherine was nee BLACK from her marriage bond to Henry MOCK. The only other BLACK family in the area was that of Adam's brother Jacob, and he had no daughter named Catherine. To have married in 1787 (plus the ages of her children) implies Catherine was born in probably the late 1760s or early 1770s and would be too old to be a grandchild of Adam, as Adam's oldest son would have only been about age 10 when Catherine was born. Since Catherine was nee BLACK, the only person in the area who was of age be her father is Adam, and he named her in his will as Catherine MOCK.
With proof that Catherine was nee BLACK, and the only person of age in the area to her father was Adam so that she would be his daughter, it increases the likelihood that Mary LIGHTS named in the same phrase was also a daughter.
Jacob SIGHTS witnessed a deed for the BLACK family. I will need to check through my files, but I think he may have also been a bondsman for one of the marriages.
The site listed in the original post doesn't list which history but stated that an Illinois [county?] history lists that Jacob Sights wife was Mary BLACK. With that "tradition" that his wife was a BLACK, again, the only BLACK in the area who could have been her father was Adam, and he names Mary SIGHTS in his will.
Do you have original records proving that Jacob SIGHTS wife was a PRESTON? I don't know of any PRESTON families living in the vicinity of where he and the BLACK family were living in NC that Jacob could have married into. Most sites I have seen also list a son Adam SIGHTS (and others) for Jacob, which would appear to be named after Adam BLACK.
I think that Jacob SIGHTS had two wives, one was Mary BLACK, and the other was a PRESTON.