To create a specific type of mirror effect in Blinn-Phong to hide the balls of light the "Glossiness" is set to 0 with the "Environment" set to 255, while for the PBR side the "Roughness" is set to 0.0 with "Metallic" set to 1.0 to create a similar mirror effect. To be clear in what the counterparts are between Blinn-Phong and PBR when creating a mirror, it is "Glossiness" that matches "Roughness", while the "Environment" matches "Metallic". Even though they act as counterparts in this type of use to create a mirror, that doesn't mean they absolutely produce the same effect when faced with a nearby object using the prim/mesh editing option in the Features tab labeled "Light" when you switch it on. Which is why I said "similar" earlier and didn't use "same".

You can see the differences demonstrated in the following picture from the provided link below.

I would suggest that the "Light" menu area should include a checkbox option labeled "Visible in Reflections", so that you could select whether or not a ball of light appears in the mirror. Plus I've seen those balls of light appear even when there is a wall separating an area when those balls shouldn't be viewable in those circumstances, which might be a software limitation or implementation issue where walls are involved, so why not give people the option of choice to hide the light in the mirror. That way this suggestion solves the stated balls of light PBR mirror issue. Where as with a Blinn-Phong mirror, all you have to do is ensure the glossiness is set to 0.0 to hide all the balls of light. Yet maybe you want some balls of light for whatever reason, then that Glossiness set to 0.0 wouldn't exactly be your best option.

Plus if you can control whether or not that ball of light is "Visible in Reflections" then maybe that would give you less things to render for each ball of light hidden in this method when looking at a mirror. Not only that but from what I can see it should be "comparatively easy" to apply this since even in the Reflection Probe menu options you can hide the avatar in the mirror by just having "Mirror (Environment)" selected. That and the fact that setting Glossiness to 0 hides all balls of light for just Blinn-Phong mirrors. In other words since those examples are currently possible as a foundation to work with, then it should be feasible to add a checkmark option to hide a ball of light directly from the light menu area, so that any prim/mesh Light can be manually chosen to either be reflected or hidden for any type of mirror (PBR or Blinn-Phong).