I'm not sure how 'not forbidding' is different than 'allowing'.
Regardless, slavery wasn't handled just through the 10th amendment. Article 1, Section 2, Paragraph 3 specifies that slaves (i.e. people who are neither free nor indianans) count as 0.6 people for determining the number of congressional representatives from a state. Because of that, I'd say that the constitution condoned slavery.
You should actually research the matter rather than parroting poorly informed talking points. The 3/5th's clause was a compromise between the Northern States that wanted slaves to count for nothing (thereby eroding the political power of the slave holding states and presumably leading to a quicker demise for the institution of slavery) and the Southern States that wanted them counted at 100%. Had the North gotten its way it's quite probable that the Civil War and ultimate emancipation of the slaves would have occurred a generation sooner than happened in our timeline.