What is the meaning of this rebus?

marcus Brutus
john wilkes Booth
Billy the kid
Bonnie parker
clyde Barrow

Black as night I can be,
Until my Mother sits on me;
Then clear as ice I become
In the rough, thank you Mum.

What am I?

Each of the following sentences contains clues to a set of three words that are spelled the same, except for one letter (each sentence describes a different set). Example: malt, melt, molt.

1) Billy discovered a cylindrical chest buried beneath the knoll.

2) The only evidence found by the detective was some adhesive in the chimney passage.

3) The bashful bovine rested on the small bed.

4) The latest fad was for people to desire the ...

1a) to wriggle 1b) a member of the British gentry 1c) a small forcible stream of liquid

2a) this color is a blend of red and yellow darkened by black
2b) the top of the head
2c) to cover with water

3a) more recent
3b) rubbish
3c) a written communication

4a) free from color
4b) a high-pitched plaintive cry
4c) although