How did I do that? ?

Another totally grey, dank day and the garden is completely sodden and lacking in colour. However, I ventured out the afternoon to practise taking macro shots, including some of a Syringa in a planter at the front door which is just starting to come into leaf.
I decided to be brave and use manual camera settings rather than the default auto. It was only when I downloaded the photos to my Mac that I discovered the camera had given them a black background instead of the blurry green grass I was expecting. I really love this effect, but I'm blowed if I know how it happened, and I bet I won't be able to recreate it again if I try!!
Any suggestions gratefully received. :-)