Black thread CAN look very severe when added as backstitch to delicate designs. Try using a darker colour of the shade already used in the design. For example, if your flower is in shades of pink, choose a very dark pink to use as the outlining colour. You should find that this gives the stitching a much softer finish.
I tried designing a picture recently, but found that the colours looked darker when stitched on the fabric than they did in the shop.
Threads do tend to look just a touch darker when worked as cross stitch than on the skein. This is because the top arm of the cross stitch creates just a very slight shadow on the lower stitch, making the finished colour darker. Although this shouldn't make a lot of difference to the finished design it is worth bearing in mind when you first choose your threads.