Blend Morphs is an older way to mix actions but nowadays, most people prefer to use Smart Bone Actions to mix actions. I'm sure it still has uses but TBH I can't remember the last time I needed to use it.
Anyway, there's really not much to using it. Here's a quick walkthrough...
1. In a new project, create a vector layer and draw a circle.
2. Open the Blend Morph Window and move it aside. Notice that at this time there is nothing in the window. This is because there are no Morphs associated with this layer yet.
3. So let's make some Morphs. Open the Actions window, click the New Action button, and name the new action Smile.
4. With the Smile action active, reshape the points in the circle into a smile shape.
5.Create another action and name this one Small.
6. With the Small action active, scale the points in the circle into a small circle. (Be sure to use Transform Points, not Transform Layer for this.)
7. Click on --Mainline-- in the actions window. This exits from actions and returns you to the main timeline. Notice that the Blend Morphs window now has two morphs in it, Smile and Small.
Just so you know, a Morph action is an action with only one keyframe, so there is no animation. Think of it as a pose target. If you add another keyframe to the action, it becomes an 'animated' Regular action and it will no longer appear in the Blend Morphs window.
8. Step into an animation frame (any frame after 0, aka, the setup frame.)
9. Slide the Smile slider to the right. The circle should morph into a smile. Enter 0 to reset it or click the Reset button.
10. Slide the Small slider to the right. The circle morphs into a smaller circle. Enter 0 to reset it or click the Reset button.
11. Now, try mixing both sliders. You should be able to make new shapes, for example, a small version of the smile shape.
12. Click Apply to create the keyframe.
If you want to redo this pose, you'll need to delete the keyframe and start over.
The reason most animator prefer using Smart Bones is because it's easier to animate with. But if you get some odd results from mixing with Smart Bones, you should look into MultiRush's Smart Bones mixing techniques here...
Mixed Smart Bones in Moho
and
Mix Smartbones Script for Moho12
The first video will show you how to set this up manaully. The second one shows you how to automate the process using a script. The script is very handy but I highly recommend watching the first video and practicing the technique manually because it's important to understand what the script is doing before you use it.
Hope this helps.