Where can I find a detailed, low-level spec for the Perfmon binary .blg file format? Or even better, has anyone written a low level, open source library (preferably in C, but any language would do) for parsing .blg files?
This won't help for looking at historical data, but if you have access to the systems running Perfmon, you may want to look at Logman. With Logman you can set performance counters AND specify the output format, that way you can just chose a format that is easy to parse. See the -f option:
-f { bin | bincirc | csv | tsv | SQL } : Specifies the file format used for collecting performance counter and trace data. You can use binary, circular binary, comma and tab separated, or SQL database formats when collecting performance counters.
As others have said if you also have historical records you need to parse you can use the Relog utility to convert existing .blg files in to a more useful format.