From the Painting Guide: You're right, it's not strictly speaking a monochromatic painting as you've used three blues. Rather it's a 'limited palette' picture, but this also serves to show that you don't need all the colours of the rainbow to create a painting.
Things to consider:
Colour mixing: Rather than working together, the three blues seem to isolate the individual bits of the painting that have been painting on one blue, such as the rocks in the foreground. Take each of the blues and paint small blocks of each from dark (100% colour with very little water) to light to see the range of tones possible from each. Although Cobalt and Ultramarine Blue will never be as dark as Indigo, each still produces quite a range on its own.
Brush marks: The leaves of the tree/bush on the lefthand side seem to be floating in the sky. It's trunk/stem where it goes behind the brush in front is hidden by the dark hill. There's a lovely crisp edge on the mountain/hill at the back where it's set against the sky, and gives the house a real sense of location.