Considering that Black Hawk Down is a story of soldiers in battle, there isn't a whole lot of themes. However, one theme that comes to mind is a saying that is printed on the cover of the book, and comes up throughout the story:
The main conflict of Black Hawk Down was the two downed helicopters. Getting to the first crash site took a toll on each of the units trying to set up perimeter around the site. Casualties and deaths piled up as the battle raged on. Getting to the second crash site was almost entirely out of the question to all of the men on the ground. They were pinned down from the Somali crowds and basically couldn't move. The only rescue to go down to Durant's crash site (the second site) were three Delta operators, who were KIA.
Other conflicts that came up during the story were the growing number of casualties and deaths during the battle and no ground convey getting to those men to take them out. A good example of this would have to be Corporal Jaime Smith, whose artery wound in the leg bled so fiercely that he bled to death waiting for the rescue convoy to get there.
A mission that was supposed to last an hour lasted over twelve mainly in part to the conflict between the rescue efforts and the men in the city. The men were taking a lot of injured and needed rescue immediately, and it didn't happen. Setting up the rescue convoy took way longer than expected, making the injured Rangers and Deltas sit like ducks in the city with no way out.