Blocked means the entire message is scrapped, not just the attachment. Future emails is a bit dodgy, I guess they mean that once the attachment is stamped as malware, the action applies across the service. Pretty much what's described in this FAQ item:
How does Advanced Threat Protection treat multiple versions of the same file? Does ATP scan duplicates? For example, if 1,000 users received the same file would ATP detonate all 1,000 messages?
After the first file is scanned, the outcome is applied to other recipients who have received the same file. For example, if File #1 was sent to Employee A and blocked, File #1 will be blocked for all other employees. File # 1 will also be blocked by reputation immediately for all other ATP tenants.