Hi Gillian :) I had the same question but was too bored to look .. but since you asked, I should find out for you :)

Apparently with ignore, you dont see their posts but they do not know that you ignore them - but by blocking a user he can understand because he will not be able to see you/your posts either.

Quoting:

You can now ignore users (the other user will not know about this and will see your posts as usual), block users (the other user will know this, as your profile will appear empty).

We actually now record that you are "not interested" in this or similar content, but we don't yet do anything about this. In the future we'll use it to personalize your feed (show less stuff you are not interested in and more of what you are interested in).

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