Turning black is known as being unstable, there are quite a few reasons for this happening. Datnoids in general are pretty shy fish and may hide and stay unstable for several weeks and it can last much longer. In my experience the bigger the dat the longer it stays unstable/in hiding when purchased by a new owner. Just give them some time and they'll eventually warm up to you much like cichlids. Until then they'll probably stay unstable, you may get many answers why they do this. Camouflage and comfort seem to be the most common reasons in my opinion. Make sure they have lots of cover so they don't feel exposed and in time they'll realize you=food, most of my dats stabled up nicely when they realized this. Stability may vary from one to another, for instance my first dat had been perfectly stable 100% of the time since day 1 while another 5" IT I got about a month ago was perfectly stable with his last owner and is still dark in my tank but has lightened considerably because he's becoming more comfortable around me and in his new environment. Hope this helps.