Microsoft has some documentation available:
Programming Guide for DDS: http://msdn.microsoft.com/en-us/library/bb943991.aspx
Reference for DDS: http://msdn.microsoft.com/en-us/library/bb943992.aspx
Compressed Texture Resources (Direct3D 9): http://msdn.microsoft.com/en-us/library/bb204843.aspx
Block Compression (Direct3D 10): http://msdn.microsoft.com/en-us/library/bb694531.aspx
The OpenGL extensions also document the block compression formats:
DXT1, DXT3 and DXT5 (equivalent to BC1, BC2 and BC3): http://www.opengl.org/registry/specs/EXT/texture_compression_s3tc.txt
LATC and RGTC (equivalent to BC4 and BC5): http://www.opengl.org/registry/specs/EXT/texture_compression_latc.txt http://www.opengl.org/registry/specs/EXT/texture_compression_rgtc.txt
BPTC (equivalent to BC6 and BC7): http://www.opengl.org/registry/specs/ARB/texture_compression_bptc.txt
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