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.