Syntax
set aSubBlob = aBlob.subblob(startIndex, length});
SemanticsExtracts a sub-blob of a given length beginning at position startIndex. Returns a new blob. The state of the current blob is not changed. If length is greater than the length of the blob, an error is raised.
SubstitutablesaBlob, aSubBlobCan be any variable or object attribute having the type Blob. 

startIndex, length

Can be any variable or object attribute having the type Integer.
Examples
set aSubBlob = aBlob.subblob(0, 2);

Example File (Builder project E2E Action Language/BaseTypes):

<your example path>\E2E Action Language\BaseTypes\uml\blob.xml