How to create black precipitate in clear liquid
I can think of iron(II) sulfide($\ce{FeS}$). There are many ways to make iron(II) sulfide but one reaction procedure will suit your purpose. It uses iron(II) sulfate and lime sulfur(calcium polysulfide) to form a clear black precipitate of iron(II) sulfide with a clear supernatent. See this e-book excerpt.

You can use other sulfides as well. See this picture (full picture here):



From left to right: silver sulfide ($\ce{Ag2S}$, black), lead sulfide ($\ce{PbS}$, black), nickel sulfide ($\ce{NiS}$, black-brown), copper sulfide ($\ce{CuS}$, dark indigo). These are formed by reacting $\ce{Na2S}$ with corresponding metal ions. If you want to fully make your solution black, then your options would be to form silver sulfide and nickel sulfide. You could also make your solution black by forming elemental mercury solution but that would be too toxic for your purpose.

Other ways include adding iodide to excess amount of bismuth(III) nitrate  in dilute nitric acid but be careful not to overdo as excess concentration of iodide will make solution orange-red(See here).

If you do not want any reaction to happen then just dump manganese(IV) dioxide in water. They are black and insoluble.