+1 for this. I have several use cases - linking to more detail from a Mind Map, or back original thoughts from a Word document, or from my PDF bible to a series of notes on a passage.
Given each block has a reference already, it’s presumably just* a matter of exposing a URL handler. e.g. logseq://open?path=xxx&ref=blockref. The main issue I can see is that Logseq would need to also open the relevant graph if not already open. It’s a bit more complex than a self contained file, like a mindmap, or a web service which can, presumably, keep an index of all blocks across all users and easily open the correct document.
Obsidian handles it like this:
obsidian://open? tells the operating system to access the Obsidian application and open a specific file.
vault=Notes tells the Obsidian app which vault to look inside.
*No idea how much coding is involved across platforms!