Cyanide
Explanation:
The cyanide ion can be detected by a blue coloration. A substance containing cyanide can be tested or identified if treated with iron salt like ferric chloride. This gives a complex known as  potassium ferric hexacyanoferrate. It appears prussian blue and thus is a color test for cyanide ion.
Formula for potassium ferric hexacyanoferrate is:
KFe₂(CN)₆