I just did a little experiment in AV mode on my Canon 5DSR with a Master Flash on body (Canon 600EX-rt) and a Slave flash in the corner (another Canon 600EX-rt).
I say the following about my observations having never shot Flash in any mode other than manual so I was a bit surprised what I saw - and it wasn't consistent when I switch to Shutter Priority.
- First of all, when in AV Mode, if you put the AV Flash mode in Camera at either 1/60-1/200 or at 1/200 (not the auto mode), and you place the ISO at any specific level (not Auto ISO), then at least with the Canon 5DSR, you will get no indication that the flash cannot expose the image properly (even if only the off camera flash is enabled) - the Exposure Meter will show middled (assuming you don't have any EC dialled in) and there is no warning, in anyway, that proper exposure cannot be achieved
- in contrast, if you switch is Shutter Priority, the at least with the Canon 5DSR, you will see a blinking Aperture until you lower the Shutter enough to achieve proper exposure (no such indication is given in AV mode which I find surprising) - but in Auto AV mode for the Shutter speed, it will drop the Shutter to whatever level is needed for proper exposure
- the last point may explain 'some' of the issue from the original question
- with the above said, even if I didn't know this, and my camera stayed at 1/60, F4, ISO 100 (when real exposure was 1/15, F4, ISO 4000), a full 6+ stops underexposed, then my flash (a 60 Guide Number Canon 600EX-rt - which is stronger than the SB700 Guide Number 28) was able to, if properly angled into a white wall and white ceiling corner - and then bouncing everywhere) able to achieve basically good room exposure (all because of the flash bouncing everywhere - it wasn't perfect but it was ok)
- so, I would argue that the SB700 should have been able to do some of this if the flash had a good white ceiling and wall to bounce off and was angled correctly to do so (which my guess is that it wasn't) - the room i used as a test was 25 feet by 20 feet
To the original post, when you do anything the first time, and something goes not perfectly, we often semi panic and can't see the solution even if it is obvious afterwards. The room was dark because you didn't have enough exposure - if your camera's meter wasn't showing you this (just as the 5DSR in AV mode with selected SS of either 1/60-1/200, or 1/200) does, then next time, turn off the flash, or turn camera to manual mode, find out what real exposure should be, and then THINK - how much of the ambient do I want to use - my answer, outside of anything that is a studio, is a lot - because that captures the environment that people will remember being in. So, once you know exposure, set your SS, AP and ISO to use at least 80% of the ambient lighting - or (-1 1/3 EV to -2/3rds EV) - that will be a good range to blend with. AFter you have got this done, then think about how you want the additional flash hitting your targets faces - the worse outcome is an on board flash using a computer's version of good ETTL exposure throwing photons at lightspeed straight at your subjects faces - it just doesn't look good unless you like the flash bomb look. So you idea of bouncing the light up into a white ceiling, using as wide angle beem as possible to diffuse the light as much as possible is best (doesn't sound like you did this) and then additionally, using an onboard flash, in ETTL, dialled back by -2/3rd EV and bounced into a close wall or ceiling or both - gives you a second and more local slight to work with.
Holy cow! You really put some work into trying to help my lost self! Thank you very much!
I definitely did not do my due diligence in checking hardly anything before trying to just throw the flash in the corner and take some pictures. I didn't realize before this that there was so much involved with using an off-camera flash. I shall certainly research, learn, and test, before trying to attempt a similar situation again.