It's a LEGO crystal brick glowing in UV strobes light, exposed on instax mini sheet inside Kiev 60 with 80mm lens and bellows.
I did a lot of such photos digitally, but what's interesting in analogue version is the color of reflection: it is different than the brick itself. I talked with friends, and our theory is that what actually makes the blue cast of reflection is the scattered UV light that bounces off the brick, then off the glass plate - old lenses let UV through quite easily and film is way more sensitive to UV than digital sensors (as they usually have UV filters in front of them...) You can see digital shot of the same setup here https://www.instagram.com/p/Befpb_5n-s8/?taken-by=konrad.sarnowski
very interesting the different results, specially how the film rendered the reflection. pretty cool