In front of them, there was a large building. It had a lot of windows, which tinted black and prevented to see what was going on inside. Like the windows most of the building was black, as were its owners' attires.
The rebels's true target was the blackguards' enlisting center, the one where Erik went to join the blackguards.
Without Erik's information, there was no way the rebels would have found where the place was, because there were thousands of such buildings scattered in New Alexandria.
When the rebels arrived, the blackguards painted black most of the places, just to throw off the rebels.
Besides, most of these buildings were filled with people, some totally unrelated to the blackguards. The people were just bait, put in just to make it looks like there were blackguards inside.