Yes, single ticks are consistent with numbers being blocked. It could also be because they left WhatsApp altogether, or because they haven't had a connection, for whatever reason (phone off, no WiFi for ages, etc.)
WhatsApp and a few other, similar, services tend to avoid wanting people to know if they have been specifically blocked or not, rather than not replying for any other reason.
Also, yes -- blocking someone on WhatsApp doesn't block someone everywhere else as well. Of course, they can also have blocked your number on their phone altogether.