Can anyone help spot what I'm doing wrong here? I'm trying to add a "simple" .5R fillet to the inside corner shown. I've tried both edge and face blends, but I'm no expert with either. It's always that last little bit that it won't apply the blend to. Ideally, the picture shown with the attempted face blend is what I'd like to achieve (or similar), but it would have to "patch" the missing surface (shown by the green/yellow adjacent surfaces).
I suspect the issue is not that the Blend shape cannot be created, but it does not know what shape to use for the capping face that would end the Blend. From the picture, it appears that if you were to extend those neighboring faces upward, they would end up curving away from the part, such that they might not fully intersect the blend face.
A trick that I have used: create the blend first as shown in the futre picture, with the planar capping face where it stops when the rail ends at the left edeg.
Next use Delete Face and remove that 3-sided planar capping face, as well, perhaps, as the 3-side shape just below it (you might not have to).
With the Healing optoin ON, see if will be able to extend those neighbors to close it back up to a Solid. If so, you can continue to work from there. If not, then it also also tells you that Edge/Face blend won't be able to either.
From there, it might get more complicated and I'd need to see the actual geometry to suggest further.
Hope that helps move you a long a bit.
It would be really difficult to comment on your query without looking at the part itself but besides following Ron and Pat suggestions can you try this option also (for your 1st image). Special Blend at Convex/Concave Y