Do bloody effusions on the face of the deceased, which increase the darkness of his skin, indicate that he has seen his place in Hell, or is that natural? And what is the best thing that can be done for the deceased after his death?

The blackness in the face of the deceased does not indicate a bad end or that they have seen their seat in Hell. What matters to God is the righteousness of deeds, not the color of the skin. Increase supplication and seeking forgiveness for them, and give charity on their behalf; for this will benefit them by consensus. Similarly, dedicating the reward of any act of worship you perform will benefit them.