Basically carbon turns the oil black and it takes only a tiny percent to make perfect oil look black. I forget the amount, but I think maybe 1/4% is more than enough to make it "look" black but still be in good shape. The carbon blows past the piston and rings during warmup while driving, so if you drive without letting it warm up.... there you go. Of course some engines are built tighter and have less blowby, some are designed to run hotter, and therefore have a looser piston which is made to expand into shape at hotter temps. Some engines have higher compression, causing more blowby with less effort. Lots of reasons why, basically warming up before driving is the best defense.