It's in the public domain. I merely wanted to make clear that I did not author the grace blessing. (It should probably be obvious, but I don't want anyone to misinterpret that I invented the blessing.)
Believe me, for things of this nature -- any more than "Now I lay me down to sleep" or "We are gather here to join this man and this woman --"
-- you don't have to do it.
In fact, you don't have to do it and you shouldn't do it for anything at all.
For someone reading a screenplay it's hard to imagine few things that would be more disruptive to the flow of the read than footnoting.
Look, for a script that I recently sold, I needed a traditional Sunday School song -- so I went on-line and I looked up traditional Sunday School songs and I found one that I thought would work in the context of my story -- and I used it.
Is it in the public domain? Don't know. Who wrote it? I have no idea. The sources that I found didn't credit it.
The script has been optioned. If and when it goes into pre-production they will put the script through what's known as "errors and omissions" -- that's a kind of insurance that movies get to cover them in case they get sued because they inadverently intrude on someone's name, or place, or produce or rights -- and they will study everything. Every person's name, every place name, ever product mentioned -- to make sure that it's all clear. And if the song is currently not in the public domain, it will be determined and if they have to pay for the rights to it -- they will.
Certain kinds of things are frowned upon - like having some big contemporary pop music hit playing on the radio.
Other things -- like they're singing "Happy Birthday" to somebody in your story. Well -- it's under copyright, it means that they'll have to pay for it -- but that's just part of the cost of doing business. If it's a valid moment in the script, so be it.
Or if they're singing "America the Beautiful" -- or whatever.
If the scene justifies it, it's not for you to figure out whether that song or that hymn or what have you, is in the public domain or not.
But if "Oops, I did it again" is playing on the radio -- they tend to be a little less forgiving of that sort of thing.
NMS