Hi All,
I had this problem last night with my iPhone 5,
Go to Settings/General/Accessibility/Physical & Motor then turn Switch Control Off.
This feature is for use with an adaptive accessory for people who may be physically disabled (like a physical button connected to the phone), iOS7 will then highlight items in turn (using a blue bar or square), the user can then select their choice (using their button or switch) when it is highlighted.
Please rest assured that the blue line is nothing to worry about and is not sign of a fault, merely part of the software.
I turned this off last night and the blue bar dissappeared. Hopefully the same will happen for you.
Good Luck.