Which of the following reaction(s) give(s) blue precipitate?

When cobalt(II) thiocyanate is added to a solution of ${\mathrm{Hg}}^{2+}$ ion, a deep blue crystalline precipitate results. Mercury's coordination number in the combination with the deep blue color is 4.

${\mathrm{Hg}}^{2+}+2\mathrm{Co}(\mathrm{SCN}{)}_2\to \underset{\text{ deep blue crystalline precipitate }}{\mathrm{Co}\left[\mathrm{Hg}(\mathrm{SCN}{)}_4\right]\downarrow }+{\mathrm{Co}}^{2+}$

When ammonia solution in water reacts with copper salt solution, a blue precipitate results.

Hence, correct options are A and C.