The purpose of using catch can setups are two-fold. The primary objective is clean out the oil vapor that is filling the airspace in the head/block BEFORE it’s routed back into the intake tract via the PCV system.
The other major benefit is it can be used to vent the excessive pressure inside the head and block to atmosphere (technically illegal for road-going cars). This helps with crank windage losses and cavitation issues, but more importantly it reduces oil burning from excessive pressures forcing oil past the valve seals into the guides, ring blow-by, etc. The blackish smoke you may be seeing occurs during initial start-up, engine braking, and hard accel? If so you can definately benefit from a PCV cleaning, and catch setup, they’re great for NA cars and turbo. I personally run a total of 4 -10AN hoses for my car, two from the head, two from the block.
Blocking off that line that connects the intake tract and the valve cover is a BAD idea. Without a proper breathing setup all you’ll be doing is pressurizing the head/block more than it already is. It is true that blocking it off will cleanse the intake charge of unburned oil, but it then forces the valvetrain/crank to work against oil vapor saturated air under high pressure.
Justin is correct about the issues with turbo cars…since the intake trat is now pressurized, much moreso than the head. Without a check valve to keep pressure from bleeding the wrong direction, that tube cannot be kept in place. For that circumstance you simply delete that line, and setup a catch can configuration that vents to atmosphere, via the bock and valve cover