Bloodraven is interesting and discussed into infinity on /r/ASOIAF. Here's a little primer.
The ThreeEyedRaven (aka ThreeEyedCrow aka the last greenseer aka Brynden Rivers aka Bloodraven in the books) is a bastard child of the Targaryen bloodline (although legitimized later by King Aegon IV Targ before he died). His nickname Bloodraven was given as a result of a birth mark on his cheek with the appearance of a raven in shape.
It is cited that Brynden was born in the year 175AC, and then he 'disappeared and was presumed dead' beyond the Wall around 252AC (making him 77 years old at presumed death). I suspected in some way or another yet unexplained (but soon in the show and books), Brynden died and was resurrected somehow as this happens all the time in the ASOIF world, or he just went AWOL- but most likely he died and through unnatural sorcery kept living- perhaps the blood of the weirwood trees. At the current time in the story, he would be around 125 years old. Contrary to what the viewer sees in the show he lost an eye in battle during the First Blackfyre Rebellion (he became a kinslayer) and typically walked around in public with just an open eye socket and no patch- although he did wear a hood most of the time and would brush his hair forward to cover the eye. Long story short- during the time Brynden was Hand of the King, several cousin-blood rebellions by the Blackfryes were quashed, there was a special council created to decide succession after a Targ Kings death, Brynden baited one such heir to KL's and had him beheaded, then the eventual king (Aenys) decided to punish him for killing that heir and gave him the options of being in the Night's Watch or death, so he took NW. Eventually Brynden became the Lord Commander of the NW in 239AC, and strangely enough 13 years later he disappeared while ranging beyond the Wall. An interesting tidbit is that when he took the black Brynden was accompanied by Maester Aemon, so there is some history there as well.
This all ties in very well with the stories of Ol Nan and how she would cite men from centuries ago - when in fact she seemed to be telling stories with a mixed hodge podge of people like Brynden, and certainly more recent in history. This is why I've always suspected that the ThreeEyedCrow was warging into Ol Nan to tell these stories -- as she was of giants blood similar to Hodor (who is her grandson). Ol Nan is just one of the tools used to get to and cultivate Bran's understanding and early skill development as the next great warg and greenseer in history.
So an astute reader might then make the connection that he (Brynden) has something to do with, if not in methods only, similar techniques of Melisandre, as he too uses a device (The ThreeEyedCros itself) to warg and 'see' events and communicate/observe. Melisandre -who is better explained in the books, has red eyes and a ruby gemstone on her necklace -- which collectively mirrors the 'seeing three red eyes' of Brynden. Prior to his post in the cave of the Children of the Forest and before being in the Night's Watch many thought that the then Lord Brynden (and Hand of the King) was a 'sinister sorcerer' of sorts. There were even sayings and songs about him having 'a thousand and one eyes'. (A 'thousand and one' is recurring term in the story when talking about greenseers).
It has been logically explained by some fans that Melisandre herself might even be the daughter of Brynden and possibly it was him who has been leading her to the North. Brynden was a suiter of Shiera Seastar, a beautiful woman who was the daughter of King Aegon IV and Lady Serenei of Lys, herself a mysterious woman. It was known that Brynden 'had relations' with Shiera, but not much else is known. There was a rivalry of step-brothers of sorts between Brynden and a man named Aegor (aka Bittersteel) as they both courted Shiera Seastar - she choosing no one but she did favor Brynden. One theory I haven't thought much about nor explored is that perhaps Melisandre is in fact Shiera Seastar herself-- or even Lady Serenei. Surely it is in someway possible, but likely easily dismissed.
Speaking of Bittersteel (aka Aegor Rivers, Bryndens half-brother), he is described as, Bittersteel was a warrior, and looked the part. He was tall and well-made, but also lean and lithe. He was only half Targaryen, so he had purple eyes, but his hair was black. This is similar to Jon Snow. Heck, even Stannis even cited Aegor - "When he fled, he swore he would return to place a son of Daemon's upon the Iron Throne. He never did."