The dip stick will come out with oil covering the tip. On a diesel engine it is normal for the oil to be dark black; the oil is designed to hold the diesel soot in suspension. If this were a gas engine it would definitely be time to change the oil just by the visual appearance of it! On a diesel you'll have to go more by age or mileage.

The oil level should fall somewhere between the notches on the tip of the stick. The notches mean:

Low - add oil

Full

If oil is at or below the Low notch add oil slowly, checking the level occasionally, until it reaches the Full notch.

Never overfill your engine with oil! Especially on a diesel. It can be pulled in to the cylinders and it could lead to a run-away engine.