blend will mix two animations together, and based on the value put into blend one animation may be stronger than the other.
For example, say you have two animations: One for walking and another for running. If you blend both animations together with a strength/value of 0.5, then both animations will play at half strength, making the model move at a jog. If you blend both animations together with a strength/value of 1, then the model will play the running animation, while a value of 0 will make the model play the walking animation.
Generally blend is used for things like transitioning from a idle state to a moving state (like walking or running) so you get nice dynamically applied animations. This makes the model look a little more natural when you just barely tap forward and the player moves forward just a nudge.
add will play two animations at once, adding them together.
For example, say you have one animation where only the legs of the model are moving, and then you have another animation where only the arms are moving. If you use a add node on these animations, then the model will both move it's legs using the leg animation, and will move it's arms using the arm animation (because it adds them together).
Generally add is used for things like having the player's legs move while playing another animation on the upper half of the body (for example, running around and swinging a sword. The running legs would be one animation, while the swinging sword would be the other animation).
At least, that is how I see the differences between them. I should add I have not tried Godot 3.1's new animation system, but I have dabbled a bit with other game engines that have animation blending and adding. Hopefully my explanation makes sense (there is probably much better explainations on the internet that explain it way better than I ever could :smile: )
