Holding blitz means you have to hit at least three fruits in one swipe and get the +3 combo message. You have to keep doing this. You can usually allow one or two fruit to drop after getting one, but on the next, you need to get +3 or more or it resets.
Once those blitzes added up to 10 (it may be at least 9), you get the +5 blitz. Then, you have to get at least three more +3 combos in a row (again, you miss one or two fruits in between each +3 combo) and you move up to the next (+10).
+30 blitz is the max. If you can maintain that throughout once you get to it, then you'll get higher scores.
You'll know when your into the "groove" as you'll hear the drums beating faster in the background and you'll see the arrows moving upwards in the background. As long as those arrows are still visible, it means you still have a chance to get at least another +3 combo to keep the blitz going. The more blitzes you can get, the longer it lasts, but you still don't want to miss any 3+ fruits or you start all over again with the blitzes.
I truly believe those 1,000,000+ scores are cheating. There's no other way around it.
RAZR HD - Unlocked