It's not accurate that the participants at the Constitutional Convention did not consider African-Americans. Many of our founding fathers were indeed concerned with slavery and saw an contradiction in proclaiming liberty in a land of slavery. The American Revolution was a turning point in discussion of slavery. Many, including Thomas Jefferson, faulted the tyranny of King George for imposing slavery on the English colonies. John Quincy Adams observed: "The inconsistency of the institution of domestic slavery with the principles of the Declaration of Independence was seen and lamented by all the southern patriots of the Revolution; by no one with deeper and more unalterable conviction than by the author of the Declaration himself." (
http://www.wallbuilders.com/resources/search/detail.php?ResourceID=10). America seemed poised to end slavery following the Revolution, but the invention of the cotton gin in 1783 reinvigorated the institution because suddenly American short staple cotton was now a viable export to British textile mills. Northern states continued their gradual emancipation plans, while slavery became further entrenched in Southern states, whose legislators took measures to eradicate all debate on the issue (e.g. the gag rule).
A historian of the American Revolution, Bernard Baylin, notes that the ideas of the Declaration of Independence were profoundly radical, even if it's intentions were not. The proclamation that "all men are created equal" spawned a "contagion of liberty" that would ultimately be impossible to contain (Bernard Baylin, _Ideological Origins of the American Revolution_). A group of slaves who met in New Hampshire in the 1790s invoked the language of the Declaration of Independence to challenge their own bondage. Women at the first convention at Seneca Falls would similarly appropriate that language to demand an end to their own subservience.
The ideas launched through the Revolution were of great importance and contained the seeds that would eventually lead to political equality. It's important to remember that eighteenth-century societies were entirely hierarchical. To proclaim equality of any kind was a profound development. That the founders understood that equality in narrow terms does not mean the values and ideas themselves are anti-democratic or unethical. I have advocated for some months now that the Democrats should reclaim the language of our founding documents and use it to critique Republican dominance and class rule. It is the implementation of those ideas that is unethical, not the principles themselves.