Adding to the body of knowledge in this thread which has greatly helped me...
I would highly recommend acetone for use in dissolving/cleaning the black death. I recently had to do a couple of mine, and had enough black death compound to try a dozen different solvents against it for efficacy, and acetone was by far the most effective.
The other outstanding agent was carb cleaner, which at least in my example was a mixture of acetone and toluene. I elected to use acetone as I was worried the toluene would be more aggressive against plastics. While acetone can also be harmful to some plastics, it didn't seem overly reactive when used judiciously (q-tip) on the injectors and electrical connectors. Gasket remover also seemed somewhat effective, but also very nasty reactive stuff, and I was again worried about all the plastic components, particularly when the injectors are still in place. The foamy nature of gasket remover and oven cleaner could be more useful once the injectors are out and the area clear, but I didn't see a need to try anything other than acetone.
I tried a couple different approaches, firstly I removed as much of the sticky mess as possible with picks and other small tools while the engine was hot(which I then used for testing). In my first approach I simply stuffed a folded paper towel in behind the injector body and carefully poured small amounts of acetone in front of the injector, which would dissolve some of the black death, and then be absorbed up into the paper towel. This was effective but slow. I later rigged up a vacuum system to just remove the acetone as as liquid. The same idea of pouring in the front, and sucking it from the rear but in a continuous process. That basically worked, but wasn't optimal as I had some material compatibility issues with the acetone attacking the vacuum tubing, which was not entirely unexpected. Polyethylene tubing probably would have fared better than the small engine fuel tubing I was using.
One thing to watch out for is adding too much acetone, as there are some small ridges that separate the injector wells, if you overflow those, you can actually move dissolved black death into other adjacent injector wells, which would be undesirable especially if you have only a very minor and localized case. Hard to tell in my case as it was more severe and was spreading pretty widely anyway with injectors 2&3 both leaking.
Once the injectors are out, it is of course much easier to use paper towels, q-tips, gun cleaning patches, and wooden dowels. I was able to achieve very clean results in the end. It also worked well in the hold down bolt holes where I would fill them up with acetone, and then insert several q-tips to soak up the dissolved muck.
Acetone does have a fairly low boiling point so for deep cleaning you really have to have a cold engine, I did most of my cleaning around the injectors in the days ahead of actually changing them out. The low boiling point can also help a bit on a warm engine to help clean the hold down bolt holes as the boiling action will blow some of the crap out of the holes. The low boiling point can be a positive as it will evaporate without any residue given time/heat.
After initial cleaning to get the majority of the accessible black death I then soaked the injector wells in Kroil which I've had success with in the past for penetrating and loosening difficult items like injectors and glow plugs etc... It was also one of the compounds I tested against the black death, and while it doesn't dissolve it, it did have noticeable effect in softening it up, which can only be beneficial to the process. Since it's not as volatile, it should stick around and soak in better, although it does of course gets blown out of the injector wells by the leaking exhaust gasses while driving.
An full list of the compounds I tried:
Paint Thinner, Acetone, Mineral Spirits, Kerosene, Gasket Remover, Brake Cleaner, Denatured Alcohol, SeaFoam, Oven Cleaner, 3M Adhesive Remover, Carb Cleaner, Kroil, Naptha
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