Blood is a squeamish subject, but leaving sacrifical virgins aside, red ochre was one of the early substances used in burial, and it is not only a staining medium it also has iron in it, same as blood.
There was one Scand. pictorial rockart on a vertical face that had been outlined in red ochre, it could still be seen today, when the atmosphere and lighting was right. So following this theory through, its not necessarily the blood but it is the colour thats the important bit,red sky at night and all that.....
and, according to roman writing, druids had to train for 20 years which presumably means they were following the cycle of the 19.8 years, or whatever of the moon.