A crossblooded sorcerer knows 1 fewer spells per spell level. And while I am pretty sure that I know the answer to this, I figured I ask.
When you pick up a new level where you only gain a single spell known (4, 6, 8, et.) does that mean that you *only* gain the spell slots with no known spells or is there a minimum of at least 1 spell known?
I suspect that you have 0 known spells with some number of spell slots, that can only be used for lower levels spells, much that I'd rather it be a minimum of 1. Although I guess you could be further penalized by saying you don't even have the slots since you don't know any spells of that level.
You have the slots but no spells known at those levels. You should learn at least one metamagic feat so that you can do something with those slots other than cast unmodified versions of lower level spells.
Like David said, you can use that spell slot to cast metamagic'd lower spells. But yes, you still have the spell slots but no spells known of that level.