du(1) that comes with FreeBSD does not understand the -b argument, and these is no way to achieve such functionality using the BLOCKSIZE environment variable. As far as I know, this option is not required by POSIX, but it's trivially implemented using a different member of the stat(2) structure and an if to separate block counts from byte counts.
The only issue that I can see is for filesystems containing files that sum up to more than 8 exabytes in size -- in this case, the 64-bit integer used to store the byte count will overflow. Multiply this by 512 for the maximum size for block-based counting. If it becomes an issue, a check can be put in to compare the addition and, if the result is less than the additive value, throw an error.
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Status: In Progress Changed: (is less than) 2014-06-01
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