The MEANING of BRR is that it is a work song. A shanty. A means to an end.
If you wanna know what the phrase means, ask him wot wrote it.
If it helps you to have an idea as to the meanings of phrases you sing, all I can suggest is to pick one.
You are as likely to be right as not, and it only matters to you unless you choose to explain it all, at length, to your audience.
In that case, you are as likely to be wrong, as not.
Sods law.
I have heard all sorts of ideas concerning meanings. Some interesting, some really thought provoking, some downright ridiculous.
But in the long run its only your own idea that matters to you when you are singing.
MY idea of Blood Red Roses is that it is a mondegreen.
The sailors of old were apt to suffer somewhat from sun, wind & weather, so the actual original correct phrase was.........
Blood Red Noses.
(An' if ya believe dat, den you is in need of more help dan Mudcat c'n offa, matey)