Which compound is responsible for the formation of a blue precipitate when ferric ions react with a Lassaigne's extract or potassium ferrocyanide solution?

Given below are two statements :
Statement $I$ : In Lassaigne's Test,when both nitrogen and sulphur are present in an organic compound,sodium thiocyanate is formed.
Statement $II$ : If both nitrogen and sulphur are present in an organic compound,then the excess of sodium used in sodium fusion will decompose the sodium thiocyanate formed to give $NaCN$ and $Na_{2}S$.
In the light of the above statements,choose the most appropriate answer from the options given below.

What is the fundamental principle of the Lassaigne test?

The blue color obtained in the Lassaigne's test is due to the formation of which compound?

Assertion: Potassium can be used in the Lassaigne test.
Reason: Potassium reacts vigorously.

Which of the following substances will give the Lassaigne's test for nitrogen?