Just, by playing around with DC Android version, I blocked a contact.
But now I can’t find a way to unblock this contact. 
Sorry, I found it in settings > Privacy.
today in Delta Chat 2.25.0 it’s Settings > Chats and Media > Blocked Contacts.
@webratte thanks for creating this topic and leaving a solution in 2019. despite outdated, it helped me to find the above solution, in 2025.
