You could trigger Apoptosis. You can trigger it by chemical signals external to the cell, triggering "cell suicide".
Apoptosis triggered by external signals: the extrinsic or death receptor pathway
- Fas and the TNF receptor are integral membrane proteins with their receptor domains exposed at the surface of the cell
- binding of the complementary death activator (FasL and TNF respectively) transmits a signal to the cytoplasm that leads to
activation of caspase 8
- caspase 8 (like caspase 9) initiates a cascade of caspase activation leading to
- phagocytosis of the cell.
Example (image): When cytotoxic T cells recognize (bind to) their target,
- they produce more FasL at their surface.
- This binds with the Fas on the surface of the target cell leading to its death by apoptosis.
The early steps in apoptosis are reversible — at least in C. elegans. In some cases, final destruction of the cell is guaranteed only with its engulfment by a phagocyte.
Source: Apoptosis at Kimball's Biology Pages. Wikipedia links added for reference.
The process naturally stars by the presence of FasL - It should be possible to accelerate it by exposing the subject to an large dose of the Death-inducing signaling complex.
The person would die before the cell destruction of the body is complete. Apoptosis causes cell death in an orderly fashion, the dead cells break in pieces and the parts get engulfed by phagocytes. Basically the body is eating its own tissue.
Starting from the surface it would make an open wound in the skin, muscle, and veins... that should cause bleeding. Of course red cells are being eaten too. Once the process reaches the internal organs it should cause trigger systemic failure and death.
If the resources to sustain this process are being given by the attacker - perhaps even the phagocytes - it could be as fast as needed to be.
Controlling it is another subject... my suggestion would be to encapsulate the biochemical agents in small membranes that would open on contact with the target tissue. Then it could be released similar to spores.
On the first stage It would go red first and skip the inflammation of early necrosis - unless you caspases for inflammation, and would you? - So, it would look as if it were a skin burn.
Once it peels off the skin it would look more like necrosis. Yet, I would expect that it would be mostly yellow for the reasons explained below.
In the case of gangrenous necrosis, the blood supply to the area has been cut out and the cells are dying. Due to the lack circulation, macrophages that would usually eat the dead cells can't reach the area. For uncontrolled apoptosis the blood supply to the area is still intact – except on the spot, once veins are being eaten, it is bleeding - and so dead cells wont accumulate, at least not until the phagocytes has reached saturation.
Also note that if the attacker is covering the target with the biochemical agent that causes this, the appearance of the process would be affected by the appearance of that agent. That is, if you want it to look black, make the agent black.
Note: I could not find examples of uncontrolled apoptosis on the sking that were not accompanied by skin cancer.