Seer: Someone who understands how their aspect works and learns things through their aspect. They are their party's strategists, having instinctual knowledge of their alternate timeline selves which they use to keep their party from making mistakes. They also have the talent of being able to hear their exiles more clearly than most. 

Blood: Bonds, trust, unity, and connections with others. Leadership and inspiration. Many blood players would be protagonists in an rpg, and friendship is their strength.

I'd had every intention of doing this right after Mage of Blood because it's a wonderful example of the differences between Mages and Seers and ended up being busy earlier than I'd expected. Highly recommend that you read the last analysis before this one as it will also serve as an addendum to the Mage of Blood analysis.

The one thing Mages and Seers have in common is understanding how their aspect works. Mage's focus on the rules of the aspect in a literal sense, they guide others to follow those rules and inform them when they've broken them, and that can easily be applied to the description of Blood that I've given above, so I don't think I need to elaborate very heavily. They know how to form bonds and keep them from breaking.

Seers are more prone to understanding how to work around and break the rules of their aspect. A Seer of Blood will essentially know how to manipulate bonds and get away with lying while still retaining trust. This could obviously be applied to someone who's good at keeping a victim in an abusive relationship, but it could also be applied to someone who's acting as a double agent of some sort. Seers of Blood would know exactly how to regain trust from someone, and a Seer of Blood at their full potential could regain trust even when it by all means has been irreparably broken.

While a Mage of Blood will have their knowledge of blood come intuitively, a Seer of Blood is one who gathers information by forming bonds with others. A Mage, one who gains knowledge directly from their aspect, is one who builds their intuition off of the bonds they've already formed, as well as from observing the bonds of others, and observing the triumphs and mistakes of leaders. Instead of forming bonds to gain knowledge, they gain knowledge through examining preexisting bonds.

So, a Seer of Blood is someone who gets information directly from forming bonds and trust with others. It's why they would work so well as a double agent. They know how to manipulate people into giving them the information that they and their team need. People tell them secrets and intimate knowledge because they see a Seer of Blood as trustworthy and as a source of authority on things.

Unlike a Mage of Blood, a Seer of Blood would absolutely be the leader of their team, in major part because of things I've already mentioned, like their ability to get people to trust them and see them as an authority. Part of what gives them their ability to strategize is that their teammates trust them, even if they only trust them to know how to take down an enemy.

Finally, as Seers retain knowledge from their alternate selves in doomed timelines, a Seer of Blood simply has knowledge on the bonds they had in other timelines and can use that knowledge to their advantage.