Hi Henning,
the problem seems to be the implicit conversion from a Blob with length 1 to an integer. Try to add a lblb_00 which is declared as "blob{1} lblb_00".
... = charA( BlobMid( lb_Block, 3, 1 ) + lblb_00 )
charA() takes a string or an integer, in this case obviously an integer and an integer is 2 bytes long.
HTH
Bernhard