Black thread on black is very difficult to see. It will, however, hide mistakes and wobbles. I love lots of different colors on black. A dark purple would be subtle and still show enough that you won't feel like you are quilting blind (which tends to happen when using black thread on black) and still camouflage mistakes well, as would any dark color like a navy. Gray would work nicely but I would stay away from any variegated because the stencils you have chosen will require a fair amount of backtracking and variegated threads look best when there is no backtracking.
Other colors that look really nice on black are just about any shade of red, from fire engine primary red to deep burgundy. Teals and turquoise also look very pretty on black. And if you want to get really adventurous you could use different colors for the different motifs.
When I did my Agave Garden
Agave Garden, the big reveal I used a lot of different colors on black but viewing this quilt may give you a good idea of how much contrast you want in the thread against the black fabric. This quilt has black, dark purple, light purple, a deep red, orange, yellow, a grayish blue and a variegated rainbow all on solid black.