Your fingering makes no sense to me. However, fingering in an exam situation is not something the examiner will be looking at. Fingering in the syllabus books is only a suggestion, although it usually turns out to be the best option for most folks. S/he only wants to hear correct, regularly played notes, so how you finger them is how you are most comfortable. Dominant 7th of C maj. could be C E G Bb, with the one black key, or could be construed as G B D F, with none. If the former, then Bb can be played with a thumb. It's usually best not to use thumb on black keys, when there's an alternative, as it's the shortest digit, and puts the hand further from the player, but as any alternative is not as good, use the thumb. Seems quite late to be considering this, with an exam a week away!