Use {{Box Blue|1====Some Heading===|2=* Some
** Stuff
}}
ie. you are providing '===Some Heading===' as the parameter of name '1'.
The presence of '=' on the value confuses the parser, so you need to
tell it the name parameter name. It also fails with urls like
www.example.org?id=1
Altenatively, you can use use {{Box Blue|<h3>Some Heading</h3>|* Some
** Stuff}} which doesn't use '='s