Whats the blackest (least reflectance in the visible range) paint or material or coating I can buy?
you might look at some thermal solar collector paint.
the color you want is butthole black.
I think that black velour fabric, such as is used in movie theaters for screen masking, will reflect less light than any paint. One such is Rose Brand.
There’s also Vantablack, but you can’t get it yet.
It’s like ‘How much more black could this be?’ And the answer is ‘none’ - none more black.
It depends how far out of your way you want to go and what your budget is. I’m imagining for most values of “blackest”, you should take a look at what people building telescopes for amateur astronomy use. It’s been years since I took a vague interest, but I recall there being some preferred solutions. My brief searching suggests that blackboard paint might fit the bill. Depending on the purpose, a fuzzy flocking might also be suitable (also astronomically-sourced).
The Gold Standard (Black Standard?) of black coatings used for vthe interior of telescopes by amateur telescope makers used to be 3M Ultrasuede Black. They don’t make it anymore, but the formula has passed through various hands, and is now manufactured in Germany by Nextel/ Mankiewicz. They have an American office in South Carolina.
The paint is actually an two-component primer called “Anthrazit” (“Anthracite”, you know, like coal), followed by a “suede” paint, which is also two-component. The material is somewhat hazardous, and has been reformulated from the 3M original, which apparently wouldn’t be allowed in today’s regulatory climate.
I’ve bought it, and use it. It is a really impressive black, and beats any hardware store flat black paint hollow. It’s an expensive solution, but if you’re REALLY determined to get a good black, this is top-of-the-line.*
I understand there’s been some interesting work with black metamaterials. Good luck trying to get your hands on that, though.
*ask me about the best white sometime. Nextel/Mankiewicz makes a suede white, too, but it’s not the absolute best in terms of its optical behavior.