There Will Be Blood title significance

I don't know that the director has ever spoken out directly about why he chose that specific title, other than what you pointed out: they didn't call it Oil! because it wasn't a close-enough adaptation.

However, I think the title is not referencing anything you see in the movie itself (at least not much). Rather, it's warning about what's going to happen as a result of the events of the movie. The discovery of oil, the rise of oil as big business, the big companies moving in and taking over, etc. will all lead to a lot of violence. We only see a tiny bit of that in the movie, the several people do die as a result of the oil beneath the farm.

The title of the movie movie reminding us of the fact that a lot of blood will eventually be spilled over this oil, and what we're seeing with Daniel is just the tip of the iceberg.

Along with the posted answer, I would point people towards the Book of Exodus 7:19 from the Bible:

Then the LORD said to Moses, "Say to Aaron, 'Take your staff and stretch out your hand over the waters of Egypt, over their rivers, over their streams, and over their pools, and over all their reservoirs of water, that they may become blood; and there will be blood throughout all the land of Egypt, both in vessels of wood and in vessels of stone.'

For anyone who remembers their Moses, this refers to one of the ten deadly plagues unleashed upon the Egyptians due to the Pharaoh not releasing the Israelites.

In the context of the film, it works quite well as the symbolism of brutal revenge for a wrongdoing.

So much of the film involves betrayal and brutal back-stabbing, coupled with furious anger, on the part of so many different characters. Given this, the title, and its relation to the Biblical verse, seems quite apt.

The best analysis I have seen regarding There Will Be Blood can be found in Terri Murray's analysis in 2007.

The "Blood" is the salt of Plainview's struggle to perceived success, the toil he survived, the men he lost, his adopted son's hearing loss, all the challenges he faced in obtaining his wealth, juxtapositioned against Eli, who did not shed blood, remained unscathed until the end, ending up broke, again seeking something for nothing, having shed no blood, except his own humiliation and ultimate demise in a brutal death. One need not look outside the film to find significant bloodshed.

Being that this turned out to be a duplicate of a question I just asked, I guess I'll pose my question as an answer:

I've got some possible references, and I'm not really sure if any of them are viable:

1. A reference to the men who died in the various mines
2. A reference to all the people Dan Plainview killed
3. A reference to the "war" that he had with preacher Eli Sunday
4. A reference to his relationship with HW (in a psychological sense)

Any and all of these could be seen as a driver of the title.