Yes, I think it is so, if I remember correctly.

Double tick without blue if no read receipts.

But single tick if blocked by other user (or not delivered yet, of course).

Sometimes it is confusing your workmate just NOT send read receipt, but somehow make a reply, I was normally shocked for the first time or second time.
You can read their last online time (if they still enabled it) without the read receipt to guess whether they have read your message or not.