It is a horribly hot day in chapter seven of F. Scott Fitzgerald's The Great Gatsby when Tom and Daisy have a brief discussion about their wedding day, which was also sweltering. Daisy remembers some things about this man, but it is Tom who remembers his name.
"Biloxi," he answered shortly. "A man named Biloxi. 'Blocks' Biloxi, and he made boxes--that's a fact--and he was from Biloxi, Tennessee."
This is a man who attended the Buchanans' wedding and then stayed for three weeks until Daisy's father kicked the man out. Neither of them knew the man, and he was brought to the wedding as a friend of a friend. [It is interesting to note that Tom is so quick to condemn Gatsby and all of his supposed "unsavory" friends, yet here is a man with a very mobster-sounding name who has no social etiquette. Clearly Tom's and Daisy's families are no different than Tom and Daisy themselves--rich but lacking in class.]
"Blocks" is the man's nickname. It is based both on his last name and city, Biloxi, as well as his profession, making boxes. It is a nickname based on sound (bi-locks-ee and box-es) rather than meaning. If you kind of "slur" Biloxi, you will hear it--"Blocks." It would have been typical of such questionable characters to have nicknames based on their professions, reputations, or other things at this time period, and this nickname is based solely on sound. It has no clearer meaning than that based on the text, though there might be something more which we do not know which added to the choice of nickname.