I suggest having such sections as Related Cases, Related Articles with internal links on the Services Pages, and sections as Related Cases, Related Services – on the Case Study Pages.

While it might be more valuable to link from the blog posts text (with right anchor text) to the Services Pages and also to the Case Studies as contextual links might have higher importance that links in sections and navigations and then pass a bigger value to the Services and Case Studies Pages.

Just make sure all internal links are contextually relevant.