Sand some of the edges of your furniture piece gently. Because you are working with black paint, it won't take much for the wood underneath to show through, so less is more in each area until you get the look you want. Concentrate on edges that would be most likely to be dinged up and worn down naturally. You shouldn't sand every edge, just here and there.
Remove some of the black paint slowly by holding the sandpaper onto the surface and using very gentle back-and-forth motions. Stop when the wood underneath just starts to shine through. Concentrate on areas that would see the most use if it were aged naturally, such as the arms, back rest or seat of a chair.
Scrape a wire brush gently across surfaces that would have seen usage from bumping or other furniture. For instance, the rungs of a chair or the legs of a dresser would have had a lot of wear and tear.
Hammer minor dents into your piece with a mallet. Don't get carried away in this process since getting over-enthusiastic could ruin the item. You are simply looking for a few artistic dings and bumps to enhance the aged look.
Wipe the piece down to remove sawdust and add a protective sealant coat if you wish it to remain the same. Otherwise, leave the sealant off and allow the item to continue to age naturally.