Blood is the Rose of Mysterious Union
I don't think anyone knows, but it sounds pretty cool. Some people think that it is a reference to Jim Morrison's Wiccan wedding to Patricia Keneally:
One of Patricia’s friends, a high priestess of a coven, conducted the ceremony, assisted by a high priest. They led Jim and Patricia through a traditional handfasting, with prayers and an invocation of the Goddess, blessings, the making of two small cuts on each partner’s wrist and forearm, and the mixing of a few drops of their blood into a consecrated cup of wine from which they later drank, a ritual stepping over a broomstick, the exchanging of certain vows, and the final calling down of the Goddess’s presence.
To Patricia, this was a perfectly natural ceremony in her religion, but Jim was totally caught up in the ritual. He gave Patricia a silver claddagh, the traditional Irish wedding ring, and she gave him a matching gold one. The officiating priestess and Patricia, in her capacity as a priestess, made out two hand-printed documents, one in English, one in witch runes. Everybody present signed, with Jim and Patricia required to mark their signatures in blood. The pair were declared wedded, and Jim fainted.
Or perhaps, it is a reference to the Catholic symbolism of a rose representing the Virgin Mary, whose "mysterious union" with God created Jesus, whose blood saved the world?
Or perhaps it is just a bunch of crazy words strung together by a guy drugged out of his mind...