The first block cipher to use a form of key whitening is DES-X, which simply uses two extra 64-bit keys for whitening, beyond the normal 56-bit key of DES. This is intended to increase the complexity of a brute force attack, increasing the effective size of the key without major changes in the algorithm. DES-X's inventor, Ron Rivest, named the technique whitening.
The cipher FEAL (followed by Khufu and Khafre) introduced the practice of key whitening using portions of the same key used in the rest of the cipher. Obviously this offers no additional protection from brute force attacks, but it can make other attacks more difficult. In a Feistel cipher or similar algorithm, key whitening can increase security by concealing the specific inputs to the first and last round function. In particular, it is not susceptible to a meet-in-the-middle attack. This form of key whitening has been adopted as a feature of many later block ciphers, including MARS, RC6, and Twofish.