With the reversal (cast on target and set new focus only if Alt is pressed, otherwise cast on Focus);
Code:
#showtooltip
/cast [mod:alt][@focus] Lifebloom
/focus [mod:alt]
Note: If you want to have it set a new focus if you have ANY modifier key pressed, just remove ":alt" from the /cast and /focus lines.
From Fitzcairn's Macro-Explain-O-Matic
Code:
Show tooltip, icon, and cooldown for the first item or spell in this macro on the action bar
If you were holding the alt key then:
Cast Lifebloom? on the currently targeted unit
Otherwise:
Cast Lifebloom? on the unit saved as your focus target
If you were holding the alt key then:
Set your focus target to the currently targeted unit