One annoying thing with humanoid animation blends is that they can look terrible if you don't take knowledge of foot-falls into account. You (almost) always want a left foot-fall to follow a right foot-fall, but arbitrary blending doesn't account for this. This results in the character sliding and shuffling their feet around during blends, instead of taking believable steps.
So, you need all the foot-falls in your animation data to be annotated so you can categorise the animation frames into the left/right foot phases. When blending out of an animation that's, say, 75% of the way from a left foot-fall to a right foot-fall, you should start the new animation at the same phase (after the first left foot-fall, and 75% of the way to the next right foot-fall).
You might also be blending in some IK to make sure that feet actually rest on the ground properly, and some animations may delegate some parts of the skeleton to IK completely. e.g. the turn-right-after-left-step animation might ask the IK system to keep the left foot glued to the floor while animating the rest of the character...
