The US Constitution was actually ambivalent in regards to the institution of slavery, in that a slave counted for 3/5 of a free citizen in terms of apportioning the votes to be assigned to a state delegation. Third paragraph, Section 2, of Article 1 of the Constitution. The 13th Amendment abolished the institution of slavery, a legal distinction which seems to elude many of those who shout “Racism!” at every opportunity. (Not you, DownInFlames)
The thing to consider is this. The Constitution may not be “neutral” nor “ambivalent” at all toward slavery, standing in the middle with zero opinion.
The Constitution may be anti-slavery with a distinct opinion. Ever considered this option?
Do not forget that the new Union had just fought a war (and another was certainly possible — see 1812) so taking a position that would have alienated the southern states and divided the Union at the 2nd worst time was out of the question. The Const said “all created equal” but the only solution was to basically say that blacks weren’t humans. This is why slavery in the US was only blacks. Everywhere else (e.g., south America) any group could be slaves.