Blitz's PRNG is broken for this purpose (known issue, current behavior will not be changed) and for encryption as it depends on the FPU, and float results will vary noticeably across processors. I recommend Kanati's excellent Mersenne Twister userlib for this purpose, which can be found here:
http://www.blitzcoder.com/cgi-bin/showcase/showcase_showentry.pl?id=kanati886903202005194825&comments=no
http://www.blitzcoder.com/cgi-bin/showcase/showcase_showentry.pl?id=kanati886903202005194825&comments=no