When copper oxide is dissolves in dilute HCl blueish green colour salt is formed. The reason for it

Copper oxide (CuO) is dissolves in dil HCl reaction is neutralization

Reaction: $\underset{\text{Basic}}{\text{CuO}}+\underset{\text{Acidic}}{\text{2HC}l}\to \underset{\begin{array}{l}\text{Copper\hspace{0.17em}choride}\\ \text{(green\hspace{0.17em}colour)}\end{array}}{\text{CuC}{l}_2}+{\text{H}}_2\text{O}$