If the number were not blacked out, all Obama would have to do is instruct anyone who was in doubt as to which one was real to send a copy of his (including the certificate number) to the state of Hawaii along with $5, and Hawaii would send back confirmation staying that it was real (and thus any other certificate that disagreed with it was fake).
In short, blacking out the number has exactly the opposite effect from what you claim.
No that is lame, because if there was a number, ANYONE could request a Letter Of Verification for all the information on the certificate.
The Hawaii State Department of Health website does not say anything about requiring a certificate number in order to obtain a Letter of Verification. It only says to provide it "if known". All you have to do is provide any of the other information from the birth certificate and they will verify it.
So apparently anyone with $5 could verify whether Barack Hussein Obama II was born on a given day in Hawaii without needing a certificate number.