If I block someone on my iPhone will I stop receiving their messages on my iPad as well or do I have to block the number on both?
If both of your devices are syncing to the same iCloud account, your "Blocked" contacts will sync across both devices.
This also holds true if you wipe either device and restore it with your iCloud contacts. I believe this is a contacts configuration so it comes with your contacts.
Blocked numbers on a device will block both SMS, iMessage, and calls, however last I knew if a voicemail is left it will still be received because that is a wireless company end receiver. Apple cannot block that feature as they aren't your provider.