Since 01st of Jan 2018 FIDE put new Blitz rules effective - here is the short version:
1) The game is NO LONGER lost with the first irregular/illegal move, but only after the second one.
With the first irregular/illegal move the opponent receives an add-on to his clock of 1 minute.
2) If both flags fell or the clock of both players indicates "0.00" (when electronic clocks are used), the player who´s flag fell first (or shows the respective indication on an electronic clock), loses the game (given, that there is still mating potential).
The long version can be found at http://rules.fide.com/.