|Black obiage from Kimono Style Book 3.|
|Black obiage and black/white obijime from Casual Kimono Life.|
And sure enough, there was an answer:
In the most intense expression of mourning, a solid black kimono tied with a black obi is worn with black obi accessories and sandals. Monochrome, unpatterned kimono can be turned into "light" mourning wear by the addition of a subdued obi and black obi cord. The black obi cord is the last vestige of mourning. When that gives away to an ordinary colored one, mourning has ended. By the same token, black can never be used as a fashion color for an obi cord.
The text says nothing of the color of the obiage except when the mourning is of high intense and paired with mofuku. So maybe the black obiage is not such a "That's funeral wear!" as the black obijime and the usage of it in fashion can be more freely. You tend to see more black obiage than blackish obijime.
The black'n'white obijime the lovely Anna Tsuchiya is wearing in the later photo might be pushing the rule of black obijime a little, as in the second picture you can hardly tell it's black'n'white except for that little white spot on the knot.
So, to my understanding now is that black obiage is ok to wear matched with a colorful obijime, but black in obijime should be used with consideration.