Lack of moisture could make the plant abort the blooms and make them turn brown; so could a fungal infection called Botrytis Blight (the sepals would look water soaked and eventually be covered with mold though). However, if the soil has been kept moist and there is no sign of fungal infection, it seems odd that they go from blue to brown. Usually, there are some other "colors" in between. Maybe various factors conspired to shorten the amt of time that it takes to get to brown. Meaning the progression from blue to brown included other colors but it happened faster than other times so you missed it. Weather, moisture, minerals in the soil... many things can affect these color changes and the times when they occur.

Hi luis, the blooms were a beautiful blue all summer (except for a few that got sun-scorched) We haven't had a lot of rain up until today but I've kept it watered. After the (blue) blooms faded, they simply turned purple. This has never happened before. So strange. You're probably right about other colors in between, as it was changing from blue to purple, I did noticed a variation of green, brown, pink. Thanks luis.