You'd think that they could fit the money needed for black projects inside the current military budget.
Budgetary issues are also part of the reason why encouraging population growth is the answer.
Encouraging pop grow is an answer to budgetary issue, that is true.
However, that is a long term proposal that doesn't do much to address the increasing gulf between what the US gov/military sees it's goals/commitments as vs what is actually sustainable in the long term.
As for the black budgets, only the Senate Intel Committee sees the redacted stuff, and that stuff usually doesn't get listed on NDAAs or budgets the public gets to see. So we have no way to know how much money is actually used on black projects, if it being spent wisely, and if the projects in question are actually going to be worth said cost.
Like we spent 19 billion or so on the 'Future Weapons System' that was supposed to be a combo of anti-tank and direct/indirect fire artillery; we have exactly 0 of them in the inventory, and 0 are in production, 0 on order, and the US tax payer isn't going to get a refund from it.
I mean I guess that would be one way the funnel money to black projects; create boondoggles that seemingly produce nothing but take a lot of tax payer dollars, then quietly shift where said money actually goes via black ink.