When engine oil is new and fresh it should be a good good amber looking in color and when the engine oil turns black it's time to change the oil.
The engine oil turns black as a result of contamination.
The oil picks up contaminates that protect the engine from wear and tear and the black oil is a result of soot, dirt and other combustion contaminants that the oil is picking up.
So it's okay if the oil turns black but if you notice your engine oil turning black then it's time for the engine oil to be changed as continuing to drive with the black dirty oil will damage the engine.
Oil eventually degrades overtime in the engine and then loses it's ability to properly lubricate the engine, however the engine oil can be recycled into new oil in some oil recycling facilities and then be resold and reused again.