hi,give them a good feed of fertilizer,,,,,do you live near the sea,only laurels do not like coastal regions ,blood fish and bone is a recomended feed, sprinkle around the base rake in and water ,,,,,,,stroll

The blackening could be from bad drainage. This plant is very susceptible to increased water in the soil and these wetter summers (and winters) seem to be taking their toll. This cold winter won't have helped a weakened plant either. In the wild, this plant grows as an under-storey shrub beneath larger trees on banks that are free drained but rich in leaf mould. The number of these that I have seen struggling in full sun, in heavy clay soil planted by councils etc. I'm not suggesting this with yours of course but root rot does sound likely from increased wet.

The other possibility if it's not increased soil saturation is a type of fungus that girdles the stem causing them to die though this is less likely.

If all else fails, grow it indoors. Not many people know that it makes one of the best house plants!