Going from 99% black to 99.999% black turns out not to represent that much more efficiency. It's the difference between absorbing 99 of 100 joules of energy vs

Going from 99% black to 99.999% black turns out not to represent that much more efficiency. It's the difference between absorbing 99 of 100 joules of energy vs 99.999 of 100 joules of energy; unless the rest of your system is effectively 100% efficient, it may not even be a noticeable change. It is very likely that other engineering concerns, not least of which is price, is going to dominate this question.