Hah, you learn something new everyday.
In c++, variables can be changed by passing them to functions, thanks to the reference notation.
This is something you need to pay attention to when reading the method declaration. const& is used to avoid copies when passing arguments, and means the arg is an "in". Plain & implies the method is supposed to modify this reference, in which case it's considered an "out" argument.
This is common practice when you want to use the return value of the method as a status report. Typically your return an int, with 0 for success and negative values for error codes. You can also use this approach if the method has to provide several results, but that style is mostly for C. In C++ you tend to report errors by throwing, and write a class/struct for return types that require several members.
As seen in the code you linked to, the style is actually confusing: the semantics do not outright suggest ReadLastBlockFile takes a reference, as it is not being checked for a status return. This kind of semantics suggest you are setting some internal value for object pblocktree instead. The following snippet would be a little easier to read:
int result = pblocktree->ReadLastBlockFile(nLastBlockFile)
if(result != 0)
{
//error handling
}
//do something with nLastBlockFile
With that in mind, the change in BU could very well be a mistake that led to a benign change in behavior, assuming the author of the change did not bother to acquire a broad understanding of this code before modifying it. He possibly saw a global variable changed by a local object and figured that's bad style, correcting for style without caring for the substance.