Document manual blacklist additions in README

Add a "Maintaining the blacklist" section that mirrors "Maintaining
the whitelist" and explains the manual-addition escape hatch
preserved by the three-way merge. Clarify that direct removals do
not work and must go through the whitelist instead.

README.md

@@ -99,6 +99,22 @@ with exact-string set subtraction:
 See [Lists](https://git.hisp.no/arr/blocklists/wiki/Lists) for the full
 pattern rules and examples.
 
+## Maintaining the blacklist
+
+Most blacklist maintenance happens through the whitelist (above). To **add**
+an entry that upstream Cleanuparr does not block -- a tracker-specific junk
+file, a new malware extension, anything else upstream missed -- edit
+`blacklist` directly in Gitea, add the line, commit. The next sync run
+detects the new line as a manual local addition and preserves it across
+every subsequent sync.
+
+To **remove** an entry, use `whitelist` instead. Deleting a line from
+`blacklist` does not work as a removal mechanism: the next sync re-adds it
+from upstream. Removals must go through the whitelist.
+
+See [Lists](https://git.hisp.no/arr/blocklists/wiki/Lists) for the full
+rules and worked examples.
+
 ## Sync schedule
 
 The Gitea Actions workflow runs every 7 days at 04:00 UTC and on manual