My experience with a local board says private sales are way down. Guns sit for sale, especially lower priced ones like cheap shotguns and .22LR. A deal is much less of a deal when you have to add in $20+ in fees and a couple hours of your time.
There have been a couple reports in the Denver paper about the number of BG checks and the number denied. The counts are low to the point of the # denied after appeal was in double digits for last year (sorry I cannot find that source it has been a few months). Those that know they are not going to pass a BG check will not try to pass one and will find other sources. Nobody can think a black market does not exist. This is catching the people with unpaid parking tickets and the like. (I have not seen a breakdown on why people were denied, only CBI has that data and it is not released. That last statement on who is being caught is my opinion)
A more cynical way to look at it is the laws are working; sales are down. If you look at the laws as an attack on legal gun ownership and not as a safety measure they make more sense. The unenforceable magazine law means you cannot buy them in state or mail order, even if you'll never get caught for violating it. The fee to pay for CBI means that even with the BG checks being a complete waste of time and money the gun owners are paying for it so lawmakers can just laugh. The mandatory BG check cannot be enforced either but nobody is willing to trust a stranger so it gets honored. The laws are working in their true intent, just not in the theme of public safety.