You can mix page tags and block tags, but for simplicity it helps to be consistent - using certain types of tags for one and different types of tags for the other. My page tags tend to be more about categorizing the type of page, while my block tags are more keywords and related ideas. But it shouldn’t matter too much.
“terrified of my graph becoming a huge tangled ball of wires.” - I’d recommend starting slow and only adding new levels of organization (including tags) as needed.