Half fill ice block trays with cold water and freeze solid. Remove from freezer and tip out the half blocks. Put a borage flower into each division, replace the half blocks and top them up with water. The flower is then trapped between the water and the ice. When the tray is returned to the freezer the borage flower will be set in the middle of the ice block. Otherwise the flowers tend to float to the top.
Or you can place the flower in a half filled tray, freeze then top with other half of water and freeze.