I love black mondo grass and although in early spring I have white snowdrop bulbs emerging from the strappy black leaves, in summer I like purple and pink as a contrast. So pink petunias and purple asters have been my favorites in past years, I may mix it up this summer and add orange Calendula as this more upright plant is less of a spreader than the petunias but more compact than the asters. I think orange and black will be a new color combo that will keep things less predictable.  In spring I transplant black mondo grass clumps to my patio pots so that there is always a bit of lacy black peeking from the summer annual displays. Keep growing, Marianne Binetti