Because of rules that are slightly different towards VIPs that normal members/staff such as the sig rules, avatar rules and user title rules plus any future features.
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Here's an idea. ;)
They look like normal users with a navy username but it's in BOLD that way people will know they're VIP but they get the normal colour. :D
Black VIP is as discreet as it needs to be in my opinion. Like MAD said a lot of (Super) Moderators would waste their time thinking a user was breaking a rule, then checking to find out they are VIP.
Ignore what I said. -.- Everything like a normal user just a bold username.