In cyanide where does the -ve sign on the C atom comes from?

The existence of cyanide ion must accompany with a cation, say, with potassium ion to form potassium cyanide.

Potassium atom releases one electron in its outermost shell to form a potassium ion, and the carbon atom gains this electron.  After gaining this electron, the carbon atom has 5 electrons in its outermost shell.  To make cyanide ion, 3 of the electrons in the outermost shell of C atom are used to make a triple bond with N atom, and the rest 2 form a lone pair of electron.  The structure of cyanide ion is then ⁻:C≡N:.  The C atom carries a negative charge because of the incoming electron which is released from potassium atom during the formation of potassium ion.

Kind of depends on how you assign that charge or if it is measured.  In effect, there are different ways of assigning charge (giving an oxidation number) to an element in a compound.  The Lewis dot representation indicates that there are 5 outer (Shell 2) electrons (plus the usual two in shell 1) making seven total electrons, against 6 protons in the nucleus, and thus C should be seen as having a nominal charge of -1.  However, the normal rules of assigning oxidation number assign C a +2 charge (the only combination of possible oxidation numbers of each element which is possible to equal -1 (which it must because the CN- ion has a -1 charge) is where N is -3 and C is +2.  N gets the negative charge because of its higher electronegativity relative to carbon.

In actual practice, the distortion of the electron cloud is such that both C and N are slightly negative (have an excess charge in the electron cloud surrounding each nucleus relative to the positive charge provided by the protons in the nucleus).  Neither "real" charge is an integer, though, so if you mean oxidation number, which nominally has to be an integer, the actual charge is not the oxidation number.

In effect, in reality, the one extra electron that gives the CN ion a negative 1 charge is partially distributed around the C and partially distributed around the N, so each atom is slightly negative in charge, and combined, those charges equal -1.

Not sure that answers your question but that is how things work.