Black Box testing is also called GUI testing. Black-box tester tests the software from the user's point of view. Black-box testing focuses on testing the function of the program or application against its specification, in order to determine whether combinations of inputs and operations produce expected results.
More on Black Box Testing:
Black Box Software Testing: By Cem Kaner & James Bach
This course is an introduction to black box testing.
BBST
Testing Overview and Black-Box Testing Techniques.
Below PDF includes - the basics of software testing, the two basic techniques of software testing, black-box testing and white-box testing, six types of testing that involve both black- and white-box techniques etc.
BlackBox Testing
Below PDF includes black-box testing and white-box testing definitions, Advantages & disadvantages of black box and white box testing etc.
BB vs WB Testing
Five Ways to Think about Black Box Testing.
Black Box Testing
Also See:
White Box Testing
Advantages of Black-box Testing
Exhaustive Testing
Limitations or Disadvantages of Black-box Testing