I liked this brief outline of the blades. If nothing else, it could be turned into a book to add ingame.

Things to add prehaps (Im not the strongest on TES lore tho but here I go)

- Since the writer is a member of the nobility, add a mention of how despite being outside the control of any but the emporer, or whomever is running the empire, in the case of when someone w/o the dragonblood is in charge (such as the akaviri regents, and only then only if they belive it actualy serves the empire and thus the legacy of the dragonborn), despite this lack of real control but by one blood line, they have tremendous influence in all courts of nobility. Prehaps add mention of 'nameless' nobels whom were foolish enough to spurn the blades in various manners (IE: refuse to assist a blade trying to go somewere, refuse to assist the blades in an investigation) suddenly have 'things' happen to them. Indicate that only rarley is it even suspected that the blades themselves acted to cause 'things' to happen to said noble, and imply that part of what the blades do is generaly end any plot against nobles, not out of loyalty or care for said noble, but because normaly any noble being removed in such a manner is bad for the empire, thus bad for the dragonborn, and in the case of when there is no emporer, bad for the legacy of the dragonborn.
- Add mention of the good the blades accomplish at times, IE: they're part spy, part assasian, party royal guard, and part Knight Errant, and will when they are able take the time to right a wrong, and protect the innocent, but only if it doesnt blow their cover or otherwise conflict with their primary objective of protecting the Emporer, then the Empire and the legacy of the dragonborn that it represents.
- Prehaps take mention of the blades from the various TES games and use some of them as refrences for writing more of this. IE: the warp in the west (the book is on the open market in oblivion, so that knowledge is kown), the Neraverine on Vvanderfal, add mention that it is rumored that the blades were somehow involved in that event, but the truth of the matter is not known, (depending on when the account is set to have been written, if its set AFTER the end of the main quest, add mention that the champion of cyrodil of the oblivion crisis was somehow connected to the blades, although the full story is not known.


Past that, feel free to make up vauge accounts but keep the following rules in account

-1- The blades ONLY deffinitive objective is to safeguard the dragonborn, when lacking a dragonborn to safeguard they protect the legacy of the dragonborn, the Emprie, as they see fit.
-2- They have a strong sense of morality. If their current duties permit, and they see the chance to help, they will do so. Usualy quite to the detrement of whomever has offended their sense of morality
-3- The nobility take the requests and suggestions of the blade as if it were the writ of the emporer himself, because usualy it is, or can be made so very easily if the noble is recalcitrant.
-4- Quite a few of the killings of nobles attributed to the dark brotherhood are infact actions of the blades. If a noble is disloyal of course he may simply turn up dead, or if he rules poorley and mistreats his people, thus besmirching the name of the Empire at whose pleasure he is granted his power, the noble may simply be 'removed', or not have a plot against him stopped.