Has someone blocked me?

Hi, I sent whatsapp messages to a friend tonight. However, I only got one tick against the message. That presumably means the message was sent to the server, but not relayed to their phone. However, since then I received a whatsapp message from this person, to which I have subsequently replied. Again, my message only has one tick against it.
Could this person have blocked me, yet still been able to send me messages? Is there another explanation?