Slytherin's Ghost

[Tonks]

No: HPFGUIDX 122795


This has probably been discussed before, since almost everything has 
as least once. But forgive an old Auror's memory.

I was rereading PS/SS and came to the part just after the sorting 
when they meet the ghosts. Sir Nick says the Bloody Baron is the 
Slytherin ghost. ""Harry looked over at the Slytherin table and saw 
a horrible ghost sitting there, with blank staring eyes, a gaunt 
face, and robes stained with silver blood." 

Seamus asks Sir Nicholas "how did he get covered in blood?"  Nick 
answers "I've never asked"

What is the story here? We now know that this is Unicorn blood.  How 
did Harry and Seamus know it was blood when it was silver? I guess 
because of his name. (The narrator told us. And JKR didn't want to 
explain anything more.)  What is the story behind the Bloody Baron?? 
Was this explained anywhere else, in the books, interview, etc.?

Tonks_op