Chemical test for ferrous ions
The test is better suited for $\ce{Fe^3+}$.
$$\ce{Fe^3+ + [Fe(CN)6]^4- -&gt; Fe4[Fe(CN)6]3}$$

The compound $\ce{Fe4[Fe(CN)6]3}$ has a specific name. Its called Prussian Blue.

If you perform the test for $\ce{Fe^2+}$, the precipitate won't be that dark. First partial conversion of $\ce{Fe^2+}$ to $\ce{Fe^3+}$ takes place. Then the above reaction takes place to give the prussian blue precipitate.

There are specific tests for $\ce{Fe^2+}$ as well. You can use Dipyridyl to get a red colouration which is due to the formation of $\ce{[Fe(dipy)]^2+}$.
Or, you can use Dimethylglyoxime to get red colouration again which is due to $\ce{[Fe(DMG)2]}$.