In this question https://mathoverflow.net/questions/76628/hilbert-class-field-of-quadratic-fields it is mentioned that if $d\equiv 1 \mod 4$ then the Hilbert class field of $\mathbb{Q}(\sqrt{-d})$ contains $\mathbb{Q}(i,\sqrt{d})$. 

Could someone point me to where the intersections of Hilbert class fields of imaginary quadratic fields is discussed in more detail?