Try putting them side by side (either horizontally or vertically) ... NOT diagonally. I did this and got 2 black roses within a week. Then when you get more than one black rose, place them side by side with each other and they can make more black roses. I used to have a TON of hybrid flowers... then went on vacation for a week and when I came back to animal crossing, for some reason ALL the hybrids had died :'(