Hello! Sounds like scale insect poo - leaves a soot like residue on the leaves below. Check under the leaves and you will see some brownish small lumps. Using gloves and bug killer, and a lot of time, spray under the individual leaves and along stems. You can also initially hose off the poo, which will be stopping light getting to the leaves, then wait a day for leaves to dry off a bit before spraying. Also, laurels can take a really hard cut back. If your bush is huge and the above impractical to do, then cut back to the trunk, and it will re-sprout though look bare for a couple of years!