The blood knot is a critical knot to have in your arsenal of fishing knots if you do a lot of joining lines together. This is as close to a "fail safe" knot that actually almost fuses lines together.
The blood knot is particularly ideal when joining lines of different diameters together. LIke when going from a braided line to a monofilament or fluorocarbon leader. Most other knots don't hold quite as well when the line diameters or materials are different. There tends to be a chance of "slippage" or weakness. Not with the blood knot.
Yes this can be a frustrating knot to learn, but it's well worth investing the time to learn it.