shared my experience. I have 3 times eaten crabs in Manila homes and noticed something in common. Whether it is fried or barbequed..i noticed it is black in there.
I then realised on spot that they did not do it correctly in killing and cleaning up the crab.
What you have described above is typical of what we Singaporeans Chinese do when we kill the crab (1) stab the crab at underbelly at the triangle belly shell (we do with a chopstick) like you say knife is also ok.
Do the same for the rest. (2)After few mins ripped off the shell and cut the crab into half. (3) Clean up the crab with a brush (the black color) and the pull out the grey gills)
After this cleaning procedure, you cook it or especially BBQ the crab is so clean inside and sweet and sumptous.

I showed my friend how I kill and clean the crab!! the correct way and no reason to be blackish in the meat..hehehhehe

Here’s an easier and more humane way to prepare a crab.

Put the crab into the freezer for about 30~45 minutes (depending on size of crab). Use a plastic bag to contain it, although usually it does not run around. Do not leave it too long as frozen crab loses it best original taste and texture.

Under the cold temperature, the crab goes into hibernation mode. After about 30~45 minutes, take the crab out. Make sure the crab is not moving (assumed dead), then do the usual thing, ie. remove the strings, top shells etc.

The normal way of plunging a chopstick or knife at bottom of thorax most of the time makes the crab struggle and have their legs and pincers naturally drop off. The killer sweats, the crab cries, together they struggle. Not a graceful way of killing a crab and not so neat for presentation for dining guests should the legs and pincers comes off before cooking.

cheers!