Blow by is exhaust gas blowing down the side of the piston rings into the sump...(worn rings/worn/glazed bores)...the soot colours the oil...I would'nt worry.
Think another big reason the oil gets dirty quick is that there's plenty of gunge in an old engine which does'nt drain away.
Flushing oils used to be common, but don't seem to be a fashion anymore...
Re: old oil...the way I understand it is the lubricating bit (molecules) can be seen as microscopic round balls, that allow surfaces to roll over each other without touching...the balls get flattened/hammered over time...