I presume that your first line is a typo? Aperture controls the amount of flash you need, not ambient.
It may just be your choice of words, but it sounds like you just have too much of a mismatch between ambient and flash. You say you are shooting in "pitch black darkness" and are struggling to get the flash power down enough so that you can still record ambient. In pitch black darkness, there IS no ambient, so I assume that you actually mean very dark, but that would still require a lot of exposure to show the scene properly. A flash head, even on minimum power, is a lot of light in such dark conditions.
What shutter speeds are you using though? Even if extremely dark, you should be able to get a suitable exposure. If you need five minutes to expose the ambient to balance the flash, then give it five minutes. So long as the background isn't moving you will be fine. Your subject will be lit by the flash, so will be sharp, just try and keep them still, and the light on them to an absolute minimum to prevent too much ghosting if they move whilst the ambient is recording.
If you find ghosting is an issue, then just shoot two frames, one of the subject with flash and the other for the ambient, then blend with a layer mask.