That's done to produce the black brick patterned grid over the balconies.
The black grid looks very nice over the white painted balcony undersides and it's best appreciated from the street level. Though I think the grid will get lost when seen from a distance with the window wall as a backdrop.
This is going to be a very handsome building.
Agreed—Tableau promises to look great. One note though; it's looking to me like the white edges are going to be the dominant feature in the end, at least from any distance, when it's clear in the renderings that the black edges are meant to dominate the design. From up close though, the black will rule.