I can't speak to how many black men actually voted when it was legal to do so, but enough did so consistently enough that they were an identifiable bloc with known partisan alignment in some states, first as Federalists and then Whigs in the main. Though we should bear in mind that this is a pretty small constituency in most of the North even before we get to an issue coming up next paragraph. There are Federalists who oppose universal manhood suffrage and argue that it would lead to blacks and white equally voting, even though blacks who could vote tended to prefer Federalists.

It wasn't quite a loophole at first. Universal manhood suffrage was not a given in the eighteenth and into the nineteenth century. Instead voting usually required ownership of a certain value of land, payment of a requisite amount of taxes, and other stuff to keep it a (relatively) well-off dudes only situation. The idea was that men of quality deserved to vote as only they were independent and disinterested enough to guide a republic. A man who was dependent could never truly be the master of his own will or passions and so would serve as either an incompetent voter or one easily led.

As long as that's the accepted system of things, the very few black men who have the cash don't appear to be all that much threat to the system...even in the slave states. (NC let black men vote into the 1830s too.) Most free blacks are poor and often getting poorer as time goes on. (Free blacks in the Lower South are a partial exception.) So in theory this is a "problem" that might solve itself over time.

But as popular democracy becomes more the accepted wisdom, and urgent, over the first half of the nineteenth century, the obvious counterargument is that "all men" really does mean all men. That implies a kind of equality that most white Americans find utterly abhorrent. Before the vote was limited anyway and not having it didn't necessarily mean a lot, but enfranchisement becoming more common made it something that more men had in common. In a way, it became a marker of community acceptance in a sense that the vote had previously lacked.

Toss that idea out in a viciously white supremacist environment, and combine it with the more populist of the two parties generally being the more openly and viciously white supremacist game in town (not that the other guys don't put up a good fight, though) and it's obvious who needs to lose. There's almost zero white constituency for anything that might lead to black officials, even at the most local level, because those would end up sending legislators to state houses and even Congress. They might hold the balance of power between the two parties and so extract all the perks of playing kingmaker. The advancement of whites is the oppression of blacks, which is a depressingly consistent thing.

You could call the property requirements a loophole; they certainly weren't designed to let black men vote. But on the other hand, there were not enough black men qualified for the vote that it appears to have been a big issue until a more popular style of democracy became dominant in the 1800s.

This all leads up to a situation where, to quote Leon Litwack's classic North of Slavery: The Negro in the Free States, 1790-1860

By 1840, some 93 per cent of the northern free Negro population lived in states which completely or practically excluded them from the right to vote. Only in Massachusetts, New Hampshire, Vermont, and Maine could Negroes vote on an equal basis with whites. In New York, they could vote if they first met certain property and residence requirements. In New Jersey, Pennsylvania, and Connecticut, they were completely disfranchised, after having once enjoyed the ballot.