This was an interesting effect. Leslie uses an interference paint straight into her Floetrol and uses that within the area of her swipe painting. She is a master of swiping with black and creates the most gorgeous color schemes. See how she really uses a minimal amount of the black paint to swipe with and still creates fabulous cells.
I think the Floetrol in the paints really help with the cell formation and the black paint is just enough to outline them and make them beautifully defined. If you’ve not tried using Floetrol in your swipes yet, give it a try! You might actually prefer it over the more expensive Liquitex Pouring Medium. Or try a little of both and get the benefits of each.