here are a few tips etc when travelling to Venezuela, the black market rate as it's an election year has exploded it's around 9.5 BsFs to the $.
The street rate is 8 maybe 8.5 if you have a decent amount to exchange, the best place and probably most safe is in the International terminal at CCS, guys will approach you as soon as you get thru customs or in the terminal, most speak some english, and offer siete 7, you say no ocho 8, usually if you have around $1000 they will give you that.
Remember it is illegal and if a National Guardsman sees you you will end up giving him everything to avoid jail, be smart and savy, make sure the guy who exchanges your $s does not call you a taxi, because odds are that is his friend and half way to Caracas, he will pull over put a gun to your head and steal everything you own.
As far as exchanging Bolivars in Colombia then back into $s, be careful ( Colombia is after Peru the second biggest forgers of counterfeit $s so carry a marker pen like they use in banks to check the $s are legit), the Chavez govt. has been cracking down on that and the borders now are very very dangerous for anyone doing multiple crossings, due in part to the illegal sale of gasoline, so make sure you stash your cash in a safe place not the usual money belt etc, I've been going there for 8+ years so I know the scoop, just be careful and don't expect any help from the cops as most are on the take too