Say I have two acions (e.g. Walk Forward and Walk Left). Is it possible to generate a new action which blends these two actions, each having an influence of 0.5 (e.g. obtaining Walk Forward Left)?
I'm not an expert, but you have to see in the NLA editor each action blend modes. I tried a simple setup to explain basic things:
Here you see two separate actions overlapping in the NLA editor. I have muted both to show that in this way you can control each individually. With both muted, you see no action at all.
If only one is muted, you see only the other action effect.
If you enable both (un-muting both), you see... only the first. Why? It depends on the blend mode, which by default is "replace", that means the highest action replaces whatever is below it.
In the following example, you can se that you can
This is a simple intro, and others will perhaps give you more help but perhaps showing your setup, actions and intended result, it will be easier to create a more specific answer...