How do you say may this union be blessed in Greek?

There isn't one single perfect translation of "May this union be blessed" in Greek, as the best choice depends on the context (religious or secular) and the desired level of formality. Here are a few options:

* Είθε να ευλογηθεί αυτή η ένωση (Eithe na euloghthí aftí i énosi): This is a fairly formal and neutral option. "Eithe" expresses a wish, "na" introduces the subjunctive, "euloghthí" is the passive aorist subjunctive of "eulogeo" (to bless), and "aftí i énosi" means "this union."

* Ας ευλογηθεί αυτή η ένωση (As euloghthí aftí i énosi): Similar to the above, but using "as" which is a slightly less formal way to express a wish.

* Να ευλογηθεί αυτή η ένωση (Na euloghthí aftí i énosi): This is even more concise and slightly less formal, omitting the wish particle entirely. The subjunctive implies the wish.

* Κύριε, ευλόγησε αυτή την ένωση (Kýrie, eulogise aftí tin énosi): This is a more explicitly religious option, invoking God ("Kýrie," Lord).

The best choice depends on your specific needs. If you're looking for something for a wedding ceremony, the religious option might be preferred. For a more secular setting, one of the first three options would be suitable.