What is blackbox testing ?

Blackboxes are the names used for the devices in planes that are expected to record the happenings in the plane and are useful if the plane crashes. However, the software blackbox is a different concept. It refers to the term of treating the software application as a box, of which you don't know the internals and test only on the basis of inputs and outputs.
There are many definitions of blackbox testing:
Also known as functional testing. A software testing type whereby the internal workings of the item being tested are not known by the person doing the testing. The tester only knows the inputs and what the expected outcomes should be and not how the program arrives at those outputs. The tester does not have details about the programming code and does not need any further knowledge of the program other than its specifications.
When black box testing is applied to software engineering, the tester would only know the "legal" inputs and what the expected outputs should be, but not how the program actually arrives at those outputs. It is because of this that black box testing can be considered testing with respect to the specifications, no other knowledge of the program is necessary. For this reason, the tester and the programmer can be independent of one another, avoiding programmer bias toward his own work. Black box tests are also the only form of test the customer is likely to understand; Therefore, black box testing is absolutely mandatory for acceptance testing. The customer must be able to understand these tests, so that he/she will know for sure whether or not you've met the contract requirements.
The base of the Black box testing strategy lies in the selection of appropriate data as per functionality and testing it against the functional specifications in order to check for normal and abnormal behavior of the system. Now a days, it is becoming common to route the Testing work to a third party as the developer of the system knows too much of the internal logic and coding of the system, which makes it unfit to test the application by the developer.
In order to implement Black Box Testing Strategy, the tester is needed to be thorough with the requirement specifications of the system and as a user, should know, how the system should behave in response to the particular action