In general, you should not specify DCB= attributes in JCL except in the few cases mentioned in the COBOL Programming Guide.
It is strongly recommended that you take advantage of features of DFSMS and COBOL that allow you to use system-determined block size, which requires no BLKSIZE specification in the JCL.
For block size, these are the recommendations:
For new files, let z/OS determine the block size. To take advantage of system-determined block size:
For existing blocked data sets, use the existing file block size:
The one case where you might consider putting BLKSIZE in the JCL is if you require a specific block size for a new file and you need the flexibility to modify that block size without recompiling your program.
In this case, follow these guidelines: