If file is really just a gzip file then try checking that it isn't corrupt

gunzip -tv file.bgzip
[download]

Looking at http://samtools.sourceforge.net/SAM1.pdf (which I think is the reference for the file format used by bgzip) it looks like the payload data (BAM) in the bgzip file is not ascii text - that would explain why you are seeing non-ascii text when you run it through zless.