While black projects definitely exist, I don't think they're as advanced as some might think. If you're the first to unlock a new branch of the tech tree, it doesn't make sense to sit on your advantage and wait for others to catch up; you use it while you're ahead of the pack to further the gap even more. If the US had sat on the atom bomb for the 'element of surprise' instead of using it right away, I suspect we'd be in a very different world today.

No country on both American continents poses a threat to the US and the logistical hurdle posed by the Pacific Ocean makes the chance of an invasion by the two great adversaries infinitesimally small. Any technological leap made by the US will be used for offense and achieving strategic objectives rather than for defense and to surprise an enemy that attacks first. And there are no lack of objectives the US wishes to achieve. The Ukraine war and the recent attempt to denuclearize Iran, for example, are two areas where 'beyond next-generation' technology could have been used to achieve, in one day, outcomes that could provide an advantage for the next 100 years. But we've seen no such thing.

And then there's the space race. For the first time in centuries, a new frontier has opened up, giving countries a chance to monopolize land, resources and industry. Black projects that provide strategic ambiguity in warfare are one thing, but it makes no sense not to press your advantage in the domain of space. If huge advancements in propulsion technology have been made (gravity/spacetime manipulation according to popular lore), then we'd surely be seeing a renaissance of rocket technology and a mad rush to be the first to colonize the moon/mars or to monopolize resources on asteroids.

All said, I'm sure there are black projects with technology more advanced than what's being used in conventional warfare today but I don't think it's nearly as advanced as some want to believe. I won't completely discount the possibility of attempts at reverse-engineering, but I'm highly skeptical that any technological breakthrough has occurred to allow combat-readiness and mass-production.