Hi springsgrace,
I'm not sure of any specific spell or blessing off the top of my head, but I believe that I can offer some guidance on creating your own. The most important thing to keep in mind when designing a spell or ritual of any kind is your intention with it. What do you want it to accomplish?
Words are the framework on which a spell rests, and I've found that the most effective ones are the ones that have the intention clearly indicated in the language of the spell. Many like to use rhymes, as it builds a rhythm that helps put them in the best mindset for casting, but it isn't strictly necessary. You can make the language as simple as a chant such as "Bless this union with light and love", and repeat the phrase as you focus on the outcome you want to achieve. Naturally, you could make it much more complicated if you wanted to as well.
Incorporate items that you associate with positive aspects of the relationship, such as a photo of the two of you that represents a good memory, a gift that your boyfriend gave you, or maybe a scent that reminds you of him. Having these items present will help bolster positive energy that you can further channel. Also you items representing the energies you'd like to call on for your blessing. If you intend to call on the elements, then you could have a bowl of salt (Earth), a bowl of clean water (Water), incense (Air) and a candle (Fire). As an example, you could pass something that symbolizes the two of you (like that photo I mentioned) over each of the elemental representations as a way of invoking the blessing. If you're hoping to call on a deity, then something that represents that deity to you. Drape your altar in their colors or have items that you associate with that deity present during your ritual.
As far as other items go, use things that have positive associations in your mind. Does sandalwood make you happily think of your boyfriend? Great, have shaving or oil present that you can easily catch a whiff of as you work. Do you have anything that matches the color of his eyes? Wear it during the ritual to remind you of him. Do you want to ensure that you'll be able to keep an open communication between the two of you? Maybe you could make a hand written letter to bless as well, then show it to him, as a way of symbolizing that you want to always be able to talk and be open with him (which is one of the fundamental keys of a good relationship).
There are endless, creative possibilities to what you could do. These are just some suggestions, but I hope they help.
