Kinda cruel, but...the best method I've found is to make three slices, the first two along both sides of the gills, then then a cut under the chin to get the artery connected to the heart. Hang it upside down and let gravity and the heart pump out blood. Do not thump it yet. Let it bleed out while it is live; more comes out. When it stops gushing and starts to drip slowly, whack it. Then cut around the wrist (all the way to the backbone) and massage blood out, while still hanging it upside down. Guaranteed no blood in the skeins, unless the fish was mishandled or dropped prior to bleeding.

If you're talking about the blood vein running the length of each skein; there's always a little blood in there. Just cut them off and wrap in paper towels then into a ziplock. Store overnight. The rest of the blood in the capillaries will be drawn out and absorbed into the papertowels. Now time to sprinkle some non-iodized.......................................