i had my car serviced 5 days ago and as part of that service the oil and filter was changed.After 2 days i checked my oil level and found the oil to be black this made me think that the oil had not been changed.I took the car back to the garage and asked them if this was normal they said it was normal as when the oil is drained not all of the old oil drains out of the engine so when the new oil is added it mixes with the old old that was left in the engine thus causing the oil to look black they said this happens with diesel engines is what they told me correct or not.

Also could you tell me if this model of car has a oil drain plug or is the oil exstractedwhere the dip stick goes.