First off, there has to be evidence that Africans traded with Native Americans before European contact with the Americas.
As far as I know, no archeological finding dating to before the Columbian Exchange begun in 1492 has been found linking Africa to the Americas - not any that has been accepted and interpreted by scientists as evidence thereof, of course.
There are all sorts of fringe hypothesis, some pretty bizarre ones, and “original research” without peer-reviewed studies going on everywhere on the internet, but then the internet accepts anything, all it takes is for someone to write a certain hypothesis down, and there will always be people looking for “extraordinary” stories of revisionism and conspiracy that are not boringly… scientific. Some of those “creative” hypothesis are built around the fact that Olmec head sculptures may look “negroid”, or that some ancient paintings have skin dark, nose wide and lips thick enough to look “plausibly African”, and that’s about it. But as far as I know the scientific community doesn’t consider the hypothesis of regular (not accidental and rare) contact between Africa and Pre-Columbian America very plausible with the current archeological evidence.
I feel like I need to add some observations due to the supposed evidences presented by other answers for a longslating pre-Columbian contact between Native Americans and Africans, or even the presence of a lot of African-descended populations among Pre-Columbian Native Americans:
1) Guanin and Caracoli are not metals, they are alloys of copper, gold and silver created by metallurgical techniques used in some incipient forms of metallurgy. It’s simply made of metals like copper, gold and silver, found in extremely abundant quantities in the Americas, there was no need to import it from elsewhere. Guanin has an eery resemblance to a similar word used in some African languages, but that's it, apart from the phonetic similarity (which can always happen by sheer coincidence) there is no evidence that Native American guanin metallurgy came from Africa.
2) The images of paintings, sculptures and the like often used to try to prove a Pre-Columbian African-Native American contact (non-sporadic contact, which is what we're talking about here, because "one time affairs" barely leave any evidence at all) all date from the Post-Columbian era, usually from the 18th century and early 19th century, and most of them clearly depict Black Caribs and similar populations of the Caribbean islands (more on that later).
3) It’s no surprise to most people that there were many Native Americans who were dark-skinned, and many Europeans, who were not used to seeing thoe peoples, compared their darker skin color with that of the closest dark-skinned people they had already met since a long time before, the black Africans. But dark skin does not equate "African", as you must know.
So, you can see commentaries by European voyagers (who were no anthropologists nor intellectuals, mind you) calling Native Americans "the blacks of this land" (of course, to European pale-skinned people most Native Americans must've looked really dark) and comparing their skin color with that of Ethiopians (though one such European author also stressed that they had "thick black hair, but not very long", which is really typical of Native Americans, not of Africans). We shouldn't project the modern meaning of terms - as, for example, "blacks" being used exclusively to denote people of Subsaharan African descent - onto the distant past.
4) We all know about the history of people like the quintessential example of the Garifuna, i.e. Native Americans intermixed with runaway and shipwrecked Africans and formed new mixed-race clans in some areas. The Garifuna are basically the brothers of the Black Caribs. Many areas of the Americas, especially in the Caribbean islands, had seen a disastrous population crash (some estimate up to 90% or 95% of the original population!), and since the Caribbean was the first region to be conquered and colonized in the Americas it was of course also the first to see a nearly complete transformation of its demographic and genetic makeup. The number of indigenous people dwindled there so rapidly that Europeans had to bring large numbers of African enslaved people soon after the colonization effort started.
Many black (in fact, mixed) Caribbean native communities came to exist from the mixing of the few remnants of Amerindian tribes with the new populations, mainly black Africans, who by the 18th century had become the vast majority of the population of most Caribbean islands, and a sizeable number of them had sought refuge in more remote areas where they formed communities that were soon become kind of the “new savages” of that land in the view of Europeans.
