Witnesses at the event tell TMZ ... employees only let a few photogs shoot Blake -- telling them beforehand that this would be a "sensitive shoot," adding they'd know why when she walks down the red carpet.
Boy, were they right! Blake's noticeably pregnant, but it's unclear how far along she is.
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Blake posted a swimsuit pic a few weeks ago -- though we don't know when it was taken -- and there was no bump in sight ... possibly intended to throw people off before the big reveal.