My understanding of one functionality of blessings is that they help one to take a moment and focus one's intent on the Creator and on one's higher purpose. This idea is especially poignant when one is engaging in what one perceives as worldly things, such as eating or using the restroom.
No blessing is made before the act of physical intimacy. At least not in any general sense.
I am looking for courses of action one can take with one's spouse, sourced or otherwise, to help put one in a state of mind that ushers the Shechina, or Divine Presence, into the bed, (so to speak). A joint recitation of Song of Songs before intimacy comes to mind, for example.
I would also accept sources or ideas for why this sort of thing should not be done, if an accompanying argument can be made that still accepts the desired end. In this vein, I could accept that by focusing selflessly, lovingly, and solely, on one's spouse, one is already making intimacy holy, and that the Divine Presence needs no special attention, for example.