An excellent expansion to a great game. Not for the faint of heart, or instant gratification types.
The difficulty rating of Just Right is mainly due to the fact that EVERYTHING is customizable. I'm a big fan of options, and its nice to be able to tilt things in one way or another, or disable something you don't like at all (such as the challenging random encounters, although if you play without these, you're missing out on a very dynamic aspect of the game, although I will warn you right now, they are VERY unforgiving... not for the easily frustrated). It would be nice to have a random tech tree on/off switch, but there's already mods for that which are 100% fully compatible with multiplayer and cause no issues, so I guess it can be forgiven.
The new technologies are quite fun and aren't just a bunch of upgrades that renders obsolete the old stuff. One of the most important in there, is the new trade options. The trade routes add in a nice extra for those who thought the empire-running of the game was too simplified. They're also very useful for those situations in which you find yourself short of worlds and thus not making as much of an income as you'd like - now you can suppliment your income with trade routes. The graphics recieved a nice little boost. The ship explosions look more impressive now, and ships with volatile cargoes (tankers, refineries and extended range ships which carry loads of fuel) will explode in rather impressive blasts that can harm nearby vessels. The new race has some excellent voice acting and imagery. Part of what makes SotS great is the work that goes into the background of the races, giving us 5 very well defined races, instead of the usual generic package of a dozen or so with tiny differences. The Zuul fit their image as very dark and evil beings, very obvious from their choice of flags and even the way their worlds look on the Tactical screen.
Multiplayer (in my opinion) remains as one of SotS's greatest strengths, and is made more diverse and dynamic with all the new options the expansion brings to the table. The Zuul are forced to be very expanionistic and aggressive, and are well equipped for the task, but their presence gives the other races a fairly impressive ship section of their own. The wealth of tactical options in SotS, increased with Born of Blood, allows for many situations to unfold in battles, as ship design after ship design is countered, updated, and countered again in massive battles that can fill space with ordinance.
If you already own SotS and enjoy it, there's no reason not to get BoB. It just gives you more. And if you don't own SotS yet, but have enjoyed 4X games in the past, be sure to at least check out the demo to get a small feel for what the game is like. An excellent 4X game that doesn't bog you down in the end game, dealing with hundreds of colonies, and featuring excellent tactical combat, including multi-fleet encounters between multiple players or AI.