Node adds physical dmg, but no tag
So no synergy with items for physical spells at all. Personally I was trying to add skill for my cold sorcs with 2 parts of Halvar set and got zero bonus from relic’s increased cold dmg after casting physical spell
Even if there is no physical tag, I would try equipping something with %inc physical damage and observe the results before and after equipping it.
But if I remember correctly, it only hits enemies once when they touch the black hole center, so even if it does scale, it probably isn’t worth it for one hit once in a blue moon.