how to have a blast xml file in a readable and understandable way
The most human-readable blast output formats are 0-4, e.g. -outfmt 0 - which if I am not mistaken is the format used by web blast. However, these formats are a pain to automatically parse.
As Cowboy_Patrick pointed out, XML is more of a machine-readable format. The tabular output format ( -outfmt 6) is very commonly used, because it is easily parsed by unix command-line tools, and its structure also allows a somewhat easy human reading and understanding.