I'm not good at blitz, but the most improvement I've had was by learning openings. contrary to the popular belief, tactics didn't do much for me, and neither did endgames. you definitely don't get the time to calculate much anything, instead you go by your instincts. train your instincts, play a lot.
watching the big boys blitz has also helped a lot. especially how they handle irregular openings, and how practically they handle endings vs. 'correct endgame play'. they'll choose the practical over 'correct' every single time, without hesitation.
and of course play, play, play, that's the most important thing. try to avoid just starting a new one after a loss, go through the game at least cursorily. especially regarding the openings.
try to make a mental note on what got you in the trouble. (taking too much risk, taking too FEW risks, intentionally weakening your king, LPDO, vague strategic things like that. they usually exist before you fall for that fatal tactic.)