In short, you cannot see whether someone is online on WhatsApp if they blocked you. Blocking is an exhaustive feature that aims to cut off all communication between a user and the person they block, and at the same time, it obscures the fact that a block occurred. You can’t see any updates or directly contact them.
In short, no, you cannot see whether someone is online on WhatsApp if you blocked them.
Similarly, they cannot see whether you are online or your last seen status when you block them. With WhatsApp, blocking is intended to obscure active and passive communication, making it difficult for people to interact even indirectly.