Glen,

just draw a triangle on paper, A = from figure to camera, B = from figure to mirror, C = from mirror to camera and A < C . Now the optical distance from figure to camera via the mirror equals B + C that's quite larger than C and even more larger than A. So when B+C is in focus then the figure at A is the most unsharp and the mirror is somewhat in between.

In Poser: render 1 without focal blur producing a sharp image in the mirror, render 2 with focal blur for focal distance B+C.

Personally I don't like the Poser focal blurred renders at all, rendedr times are far too long and quality is far too low. I just make the sharp render plus a Z-depth map and use the latter for a mask on the photoshop blur effect - in your case while masking out the mirror reflection to keep that one sharp. The z-mask should be inverted to give most blur to the most nearby objects in the scene, and could be adjusted in contrast and brightness to regulate the effect your want.

"Never do in 3D what can be done in 2D" is the motto. 

As BB says, Poser blurring is a somewhat post effect. Make a reflective steel ball with blurred reflectivity and look at that ball through a (blur-less) mirror as well. Same for skin scattering. All those effects work on rays directly towards the camera only. One just needs other tools for better results.

Have fun. 

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though