Never heard of Blackrocks. Evidently they are a hybrid cross developed by a specific hatchery in Scotland. Lohmann Browns were developed in Germany and should give you fairly similar chickens to the Blackrock, just a different color.
The Lohmann Browns and Blackrock will probably give you a few more eggs than the Buff Sussex and will probably have smaller bodies. I'd expect their personalities to be pretty similar. The Buff Sussex is more likely to go broody than the other two but a lot of that depends on the individual.
I don't see that you can go wrong with any of them. Wouldn't it be fun to get one of each and have a more colorful flock. Or if only two the Lohmann Brown and Blackrock.
