You should have all the control you need in the Onyx software....but any adjustments you make will likely require re-profiling the device.
I think first you need to determine whether it's truly a black ink issue or whether it's a total ink limit issue either in the Onyx RIP or the profile that you're using. If you're using an ICC color-manged workflow, the chances are that what you're perceiving as "black" is actually a 4-color build meant to reach the maximum density that's achievable by your printer+media combination. The optimum total ink amount is a setting that achieves the maximum achievable density with the least amount of ink.
You will likely have to build a new media calibration from scratch and determine the optimum per-channel ink limits as well as the total ink limit and then build a new profile for that media calibration.
Terry
Terence Wyse, WyseConsul
Color Management Consulting, G7 Certified Expert