What you want to do is impossible. You're trying to use a Blinn (or any other Specular node) to represent that light is available from that direction, and modulate the reflection map based on shadows falling from that direction. But the Blinn node can only turn on within a hemisphere. That's why you get the sharp edge - it simply cannot "see" the light beyond the "horizon".

When you increase the Blinn and make it really wide, it fires at impossible angles, right at 90 degrees between the surface normal and the light source.

If you try to use multiple light sources then they overlap and you lose the shadows.

This technique is only going to work at limited angles and camera viewpoints and lighting scenarios. It's no different than the problem with using a camera lens that has problems with flare. You can't point such a lens anywhere near the sun. Other, better, more expensive lenses have multiple specialized coatings to prevent this and allow shooting into the sun. That doesn't make the cheaper lens useless - it just limits the utility.

So, too, there are other, specialized, more expensive renderers that can do soft speculars with environment maps. Poser doesn't have this but it's actually easy to do. If they would let me work on the source code for the renderer, I'm sure I could fix it. That's not what I'm working on. :)

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