During the Battle of the Blackwater, Ser Boros and Ser Mandon Moore remain at the side of King Joffrey as he oversees the city's defenses alongside Tyrion Lannister, the Hand of the King. When Queen Cersei learns that the forces of Stannis Baratheon have reach the city are beginning to invading it, she orders Joffrey to be brought into the safety of the Red Keep. When told that he has to leave the wall at the behest of Queen Cersei, Joffrey, who has never shown much courage in the face of adversity, complies and orders Ser Mandon and Ser Boros to aid Tyrion and fight on in his stead. They stand behind Tyrion as he gives a speech to rally the troops. 
In the A Song of Ice and Fire novels, Boros Blount is a member of Robert Baratheon's Kingsguard. He is a short-tempered, ugly man with a broad chest and short, bandy legs. He has a flat nose, jowls and grey hair. Ser Jaime Lannister considers him an angry paper shield. Many characters throughout the series think of Blount with contempt due to the man's cowardice. Like Ser Meryn Trant, Varys considers him one of "Cersei's creatures".
Meryn, along with the other five Kingsguard members present, openly laughs when his Lord Commander Barristan Selmy is unlawfully dismissed from service.
In the books, it is Boros Blount the one who carries out Joffrey's orders to undress and beat Sansa Stark instead of Ser Meryn. He's also absent during the Battle of the Blackwater, having been dismissed for cowardice after yielding Prince Tommen to Tyrion's agents when the Prince is smuggled out of King's Landing to protect his life from the impending attack by Stannis Baratheon.