Flags are supposed to represent something about the place and people which they are flown over. Many of the Southern state flags were designed from, by people who participated in, or to remember the Confederate States of America (CSA) during the Civil War:

• Alabama, based off the Confederate state infantry battle flag
• Arkansas, three blue stars below the state name to represent the three countries which controlled it before statehood (France, Spain, the U.S.) and one blue star above the name to represent the Confederacy
• Florida, similar to the Alabama state flag, suggested by Governor Fleming, who was enlisted in the Confederate army during his youth
• Georgia, currently mimics the "Stars and Bars," the original of the three flags of the Confederate States of America (not Dukes of Hazzard one you're probably thinking of).
• Mississippi, incorporates the Confederate battle flag into a mimic of the "Stars and Bars."
• North Carolina, closely mimics the "Stars and Bars"
• Tennessee, resembles some elements of all three flags of the CSA.

Many flags designed with a blue background satisfy a few things:

• flags flying similar flags give the impression of unity, much like many African countries use the Pan-African colors of red, green, and yellow or red, black, and green, while flags of countries once part of the United States of Central America have two bars of blue sandwiching a bar of white
• out of the three colors making up the national flag, the deep blue found on the national flag most objects placed upon it to stand out greater than the red on the United States national flag. A white background on a flag is known as the international symbol of surrender

In regards to some flags that are very different from the rest, take a look at their heritage:

• California, the words "California Republic" and star imitates the Texas Republic ("the lone star State"), a bear being a symbol of pride seen throughout the state in the 1800s
• Hawaii, eight bars to represent that eight islands of Hawaii, with the British flag in the corner to honor Hawaii's friendship with the British
• Maryland, bears the arms of the Calvert and Crossland families. Calvert is the surname of the Lords Baltimore which founded Maryland. Crossland was the family of the mother of George Calvert, the first Lord of Baltimore.
• New Mexico, incorporates the colors of Spain to represent the Spanish explorers who founded the area into the Zia, an ancient sun symbol from the Native American people that inhabited the area.