I didn't test it myself, but my understanding is that content is never shared indirectly, but always fetched from the original instance. And therefore I understand @Gargron in that way that any user on an instance you have blocked personally cannot see or interact with any content of you, it never gets to that server, even if it was shared indirectly (and of course as long as it's not a screenshot).
If this is not the case, it seems a bug to me which should be reported.