Hi Jason,
I'm not sure if you CAN properly tie a blood knot using backing and fly line? I would think the difference in line diameter would be a headache. Would it hold??? Probably. Could you tie it?? Never tried. Do yourself a favor and try to learn either the Albright or Nail knot. A little trick with the nail knot.....use a piece of a ball-point pen ink tube (obviously, without the ink
) to feed the backing thru. Works for me. Leave the blood knot for tippets. Glad to help with the backing and line. Happy New Year to you, too.
Ken
"Fly fishing is the most fun you can have standing up" Arnold Gingrich