Answer:
If your child is drawing in black there is nothing to be worried about. There are several explanations of this, one of which could be related to your child.
1) Sometimes children are so involved with the contours of the drawing, its silhouette, details and specific attributes that there is no room left for color, they just completely ignore it. And that is not at all bad, on the contrary these children have an ability to concentrate, attentively follow the curvature of a line, and color does not appeal to them and seams boring and unimportant. In spite of that, these children need to be introduced to the color, but not to the whole rainbow at once, but in a sequence, starting with the favorite color, associating that color with the most frequently drawn objects. And yet it is important to develop a feel for the color, as with their observational skills, their development can stop on the level of contours.
2) Other children are personalities that have a hard time getting used to something new, but once they are comfortable then that is for a long time, for instance to the black color. Again, that is not bad, children usually consciously choose their preferences, and once decided keep on developing this functional attitude. Would not you agree that this is better than not having a personal style at all? In other words these children have a great potential, which needs to be put to work, helping the children to uncover the beauty of other colors.
3) Some children while in a sad mindset express their feelings through a preference to black. With time children can start using this behavior to their advantage and that can even become a habit. Adults may try to improve this situation by paying a lot of attention to buying colorful items and toys, constantly asking their children if they like it or not. But be careful, this might trigger an opposite reaction by inadvertently pushing children towards retaining this behavior as advantageous for getting their way with parents.
4) There is a category of children with gentle character that let their troubles, family problems, feelings of guilt and inferiority to settle deep in their consciousness, and later these feelings come out through monochrome, black and white drawings. If you look closer these will be more often unfinished artworks. With time these children in fact begin to see the world in shades of gray and might be surprised when someone tries to correct them. It will be easier for these children to color their world by practicing drawing in a group with other children, where they, as members of a group, can receive advice, assistance and fresh ideas.