No, you can't chat with a friend on WhatsApp if either of you has blocked the other. When someone is blocked on WhatsApp, several features become restricted:
Messages: Messages sent to a blocked contact will not be delivered.
Last Seen and Online Status: You won't be able to see the blocked contact's last seen or online status.
Profile Updates: Any changes to the blocked contact's profile photo, status, or other account information will not be visible to you, and vice versa.
Calls: You can't make voice or video calls to a blocked contact.
How to Unblock:
If you wish to resume chatting with the blocked contact, you'll need to unblock them first. Here's how:
Open WhatsApp and go to 'Settings'.
Tap on 'Account', then 'Privacy', and then 'Blocked Contacts'.
Find the contact you want to unblock and swipe left (iOS) or tap and hold (Android), then tap 'Unblock'.
After unblocking, you can resume chatting, but keep in mind that messages and status updates sent or made during the time the contact was blocked will not be visible to you or them.
Final Thoughts:
Blocking on WhatsApp is designed to be a comprehensive way to cut off contact. If you wish to communicate again, you'll need to unblock the contact and both parties will essentially be starting anew.