Who is a danger to blue whales?

Humans. Due to extensive and uncontrolled hunting. Blue whale hunting was banned in 1966 by the International Whaling Commission. But, by 1970 330,000 blue whales had been caught & killed in the Antarctic, 33,000 in the rest of the Southern Hemisphere, 8,200 in the North Pacific, and 7,000 in the North Atlantic. The largest original population, in the Antarctic, had been reduced to 0.15% its original count.