First of all let me admit that I have zero experience exchanging Colombian Pesos (COP) to Bolivar Fuerte (VEF is a more common term than BF), but let me make a guess...
It's actually a business in Colombia to take money out of an ATM, exchange it to dollars, then take the USDs out of the country to deposit into a USD account. If you time it right you can easily make an instant 15% to 20%. (I did very well doing this last winter.)
Thus, because USDs are so cheap in Colombia (gotta get rid of that drug money somehow) I can't imagine that you wouldn't do way better converting your COP to USD, then+ into VEF. (USD to VEF exchange rates on the black market are, as you mentioned, +very lucrative right now.)
Throw in the fact that Venezuela and Colombia are on very poor terms politically so I imagine the Venezuelans will screw you on the COP exchange rate.
Again, this is only a guess. I'll be interested to hear from someone who actually knows what they're talking about...
In any case, good luck and safe travels.
Cheers,
Terry