First, You need to find out "what the prox switches are" Are they NPN or PNP. What is there normal operating voltage. Most switches work at 10 - 24 volt DC. Your color codes are correct. the Black lead is you switch lead. It will return DC- (NPN) or DC+ (PNP) That will be your return (input signal) to your your control. 1f you run the switch on 10 - 24 volts that is what will be sent to the BoB. You need to run it through an Opto / Isolator.
This will allow and isolate the use of the higher voltage and protect your PC port.
If you have a higher end BoB, you might have the isolation built in. BE SURE OF WHAT YOU HAVE! or you will let the smoke out of your motherboard. http://www.pmdx.com/PMDX-105
I hope this helps! If you need more info just let me know. Or Contact http://www.pmdx.com