I noticed a paper has a bluish tint under mercury lighting, I was told that's because it contains some fluorescent additives which transform the UV into blue light.

I thought the paper could be used as a reflector for a mercury fixture, increasing the light output. However it is a bit impractical (can get wet, scratched, etc).

I was wondering, are there any types of paint which contain fluorescent additives like paper?

Most white acrylic paint bases contain Titanium dioxide, which is a whitener.

I would think that a reflector would provide more light than any flourescent coating, as these work by absorbing light photons shorter than a certain wavelength and then re-emit them at a characteristic colour, the difference in energy being dissipated as heat. This would actually reduce light output as opposed to a reflector that is going to reflect all incident light with only a small loss.

The only use of a flourescent material is that it will provide additional colours into the light source, but at the expense of absorbing some of the incident light. Clear tubes are slightly brighter than coated, but, because the coating in the tube is on the inside, it gets excited by the UV light that the glass normally absorbs.

Well, the point is that not ALL of the UV mercury light gets transformed by the bulb phosphors and a part of the UV actually escapes out of the bulb.

I think a paper can be thought of as a nearly perfectly white object, so it absorbs just very little visible light. However, it absorbs the UV and actually adds visible blue emission, which e.g. aluminum reflector can't do.

In my opinion it is also somewhat easier to design a wide angle flood light (which is what I want to do) with a "diffusive reflector" (such as the paper) than with "reflective reflector".

The paper is by far not the best reflector, an aluminium sheet would do much better job. And as second, the lamp radiate a lot of heat, so the paper would turn brown very soon, or even it might catch fire...
In the paper are used whiteners, converting UVA into blue/green, so make the otherwise brown paper appear white. But for use as reflector - the paper is still "brown", so absorb a lot of blue.