What does the title of the film "Blood Diamond" (2006) refer to?

'Blood diamonds' are mined in rebel held areas of a country and the proceeds are used to purchase weapons and resources to militarily overthrow the legitimate government of the country where they are mined. This has caused significant hardship and deaths in the civilian populations of many of the countries where the practice exists.

When polished, the diamonds are virtually indistinguishable from legitimate gems and buyers are unaware of their provenance. Diamond dealers such as De Beers Consolidated Mines came under pressure from the U.N. to properly document the source of their stock purchases and (some say) the global trade has significantly decreased since the 1990s.

According to the Oxford English Dictionary the earliest use of the term was in 1999 in 'Press Release Foreign & Commonwealth Office.'

https://www.britannica.com/topic/blood-diamond
https://www.oed.com/dictionary/blood-diamond_n?tl=true