This is the bytestring builder that is debuting in bytestring-0.10.4.0, which should be shipping with GHC 7.8, probably late in 2013. This builder has several nice simplifications and improvements, and more out-of-box functionality than the older blaze-builder.
Note that this package detects which version of bytestring you are compiling against, and if you are compiling against bytestring-0.10.4 or later, will be an empty package.
This package lets the new interface and implementation be used with most older compilers without upgrading bytestring, which can be rather problematic. In conjunction with blaze-builder-0.4 or later, which offers an implementation of blaze-builder in terms of bytestring-builder, this should let most people try the new interface and implementation without causing undue compatibility problems with packages that depend on blaze-builder.
GHC 7.6 did debut an almost identical interface and implementation, but with slightly different module names and organization. Trying to re-export/rename the builder provided with 7.6 did not turn out to be very practical, because this interface includes new functions that rely on Builder internals, which are not exported in 7.6. Furthermore, these module names should be deprecated in 7.10.