Dark light photography is the art of making your subject stand out against a tonally dark or indeed completely black background.
Although in the picture above the background may appear to be black, it is mostly tones of vey dark blues and greens but the important thing is that there is enough of them in the image and that they are sufficiently dark to make the in focus parts of the white lace hydrangea really stand out and imbue the whole thing with a lovely 3D pop.
In the picture above the background again isn't black but very dark green if you look carefully enough. In this case there is not so much of a dark light background as there is in the first image but I think there is still sufficient to make it work and also pop well.
This photo is really in your face because of its combination of primary colours, strong directional illumination and the dark light background; pow.
The above picture illustrates a very different application of dark light. This time, instead of large blocks of dark light in background, it is interspersed rather within the out of focus leaves behind the main subject. The essential thing here is the leaves themselves are tonally similar to the black of the dark light behind them. This is critical otherwise this picture would not work.
So the art of dark light, making the colours pop and the subject stand out, there you have it.