I'm doing something very similar to what you do here. I'm trying to take it one step further and derive the actual sql statement from the BlockingSQLHandle. When I take the handle and plug it in manually like so, all is great:

select * from sys.dm_exec_sql_text (0x03000d004884ee666fb67301b9a100000100000000000000),

but when I try to do it all in one step like the following I get an error:

select

WaitTime = cast( textdata as xml ).value( '/blocked-process-report[1]/blocked-process[1]/process[1]/@waittime','varchar(20)' ),

BlockingQuery = cast( textdata as xml ).value( '(/blocked-process-report/blocking-process/process/inputbuf)[1]','varchar(200)'),

BlockingSpid = cast( textdata as xml ).value( '/blocked-process-report[1]/blocking-process[1]/process[1]/@spid','varchar(20)' ),

BlockingLastBatchStarted = cast( textdata as xml ).value( '/blocked-process-report[1]/blocking-process[1]/process[1]/@lastbatchstarted','varchar(20)' ),

BlockingSQLHandle = cast( textdata as xml ).value( '/blocked-process-report[1]/blocking-process[1]/process[1]/executionStack[1]/frame[1]/@sqlhandle','varchar(50)' ),

BlockingStmtstart = cast( textdata as xml ).value( '/blocked-process-report[1]/blocking-process[1]/process[1]/executionStack[1]/frame[1]/@stmtstart','varchar(50)' ),

BlockingStmtend = cast( textdata as xml ).value( '/blocked-process-report[1]/blocking-process[1]/process[1]/executionStack[1]/frame[1]/@stmtend','varchar(50)' ),

BlockedQuery = cast( textdata as xml ).value( '(/blocked-process-report/blocked-process/process/inputbuf)[1]','varchar(200)'),

BlockedSpid = cast( textdata as xml ).value( '/blocked-process-report[1]/blocked-process[1]/process[1]/@spid','varchar(20)' ),

BlockedLastBatchStarted = cast( textdata as xml ).value( '/blocked-process-report[1]/blocked-process[1]/process[1]/@lastbatchstarted','varchar(20)' ),

BlockedSQLHandle = cast( textdata as xml ).value( '/blocked-process-report[1]/blocked-process[1]/process[1]/executionStack[1]/frame[1]/@sqlhandle','varchar(50)' ),

BlockedStmtstart = cast( textdata as xml ).value( '/blocked-process-report[1]/blocked-process[1]/process[1]/executionStack[1]/frame[1]/@stmtstart','varchar(50)' ),

BlockedStmtend = cast( textdata as xml ).value( '/blocked-process-report[1]/blocked-process[1]/process[1]/executionStack[1]/frame[1]/@stmtend','varchar(50)' )

from fn_trace_gettable (N'd:\trace_blockedprocesses_2013-08-19_0400.trc', default)

cross apply sys.dm_exec_sql_text(cast( textdata as xml ).value( '/blocked-process-report[1]/blocking-process[1]/process[1]/executionStack[1]/frame[1]/@sqlhandle','varchar(50)' )) AS st

where eventclass = 137

order by BlockedLastBatchStarted

anyone else seen this?