It’s a hidden feature which can be enabled by following these steps:
Step 1: Open your account on Facebook.
Step 2: Click on the “Chat” option which is present on the lower right side of your profile.
Step 3: There you will find two options. Choose “Friend List”.
Step 4: There you will find an option of creating a new list. Name it, for example, “Blacklisted”.
Step 5: Now, select or write the names of the people with whom you don’t want to chat.
Step 6: Set the status as offline for this chosen list.