The black can is called a "puke tank", and is used for blowby and
oil forced out when roadracing. If you have the stamped steel "valley pan" under your intake, you can usually run a PCV valve in the rear location, as the valley pan keeps the
oil under it for the most part so just blowby goes out the rear hole. If you're running hydraulic lifters, and especially hydraulic roller lifters where the valley pan won't fit, it's best to block that opening and run the PCV valve from a valve cover because too much
oil is flying around in the valley area, causing the PCV valve to suck oil as well as blowby.
Dan