Is there any custom function or a property in a gui that can be used to lose focus while you're focused on a textbox?
TextBox:ReleaseFocus() was added back in June on the 25th. Just thought I'd add it as an answer for anyone who dug this question up at a later date. Here's more info on ReleaseFocus:
http://wiki.roblox.com/index.php?title=API:Class/TextBox/ReleaseFocus
if you're looking for a method to Unfocus yourself from a textbox, there is none currently available.
This might be what your looking for, im unsure http://wiki.roblox.com/index.php?title=API:Class/TextBox/FocusLost if you mean it blurs, then i dont think theres a funtion for that, might have to use images instead