BLK files are basically S16 files with a slightly different header, and with slightly incorrect offsets due to the different header. They contain 128x128 sprites which are the 'blocks' which make up the background.
The header is as follows (all values are little-endian):
|flags||32-bit integer||only the first bit is used; if set, the image is in 555 format, if not, the image is in 565 format|
|backgroundwidth||16-bit integer||the width of the background, in blocks (not pixels!)|
|backgroundheight||16-bit integer||the height of the background, in blocks (not pixels!)|
|numframes||16-bit integer||the number of sprites in the file (must always equal backgroundwidth * backgroundheight)|
Then, for each sprite:
|offset||32-bit integer||offset of the sprite data from the start of the file, minus 4.|
|width||16-bit integer||width of the sprite. must be 128.|
|height||16-bit integer||height of the sprite. must be 128.|
The rest of the file consists of image data at the file offsets defined in the header. Sprites are stored as an array of horizontal scanlines of the sprite. Each pixel is represented as a 16-bit little-endian integer in the format specified in the header.
The sprites are sorted first horizontally, then vertically (so in an image which was 8 sprites (ie, 8*128 = 1024 pixels) wide and 2 sprites high, you'd get 8 sprites representing the first row, and then 8 sprites representing the second row).
For the Macintosh Creatures Evolution Engine, BLK files use big-endian values instead of little-endian ones. These files can be detected by comparing the first 32-bit little-endian flag value to 0x1000000 (i.e.., the 32-bit big-endian encoding of 0x1). They use the RGB555 format.