XML format is generated by the -m 7 parameter setting. Currently there are two XML formats, one generated by the old BLAST engine (-V T) and the one generated by the new BLAST engine (-V F). The older format was described by the corresponding .dtd and .mod files. More information is at ftp://ftp.ncbi.nlm.nih.gov/blast/documents/xml/.
One characteristic is that for search with multiple queries, the resulting XML file contains multiple XML files, one for each query. XML files generated by new BLAST engine (-V F) will be different. It resembles that for blastpgp and megablast XML output - individual queries are placed under Iteration entity instead.