Nettle Leaves Turning Black

I recently received a dozen nettle plants that I transplanted in potting soil and after a few days I find that the edges of the leaves are turning black. Please tell me what is wrong and how I may help them thrive.

Your problem may be heat. You live in California, USA, and in June the temperatures are probably pretty high. The plants were disturbed when you transplanted them and the damaged roots can't keep up with the demand for water. The edges of the leaves are farthest away from the source of water and therefore are more likely to get "cooked" in the heat. If the plants came from Richters, they had the additional problem of adjusting from our wet and cool growing conditions (for this year) to your vastly different ones.
To help them adjust and recover, keep them shaded for about a month and then gradually expose them to increasing levels of sun, but half sun should probably be the maximum. Keep them moist at all times and keep the soil rich and humusy. In nature they grow just about anywhere, but thrive best in semi-shade in humus-rich, moist, well-drained soil.