Metablasts modifiy your blast schticks without changing the basic effect. Regardless of how many metablast schtick you use to modify a Fire blast, it is still fire, though it may no longer burn. A fire blast that Drains Constitution can be visualized as choking smoke, but it is still resisted like fire.
Not all blasts go well with all metablasts. In the description of each Blast Schtick there is a list of what metablasts go with that particular blast. This list is in no way immutable, but you should consult your gamemaster before going outside it. Some metablasts have other restrictions, such as Focused that is available to certain blast schtick types (such as Items schticks) and not others.
Buying Metablasts
Each metablast is bought as a separate blast schtick. Once bought, it only applies to one of your blast schticks.
Example: Gothrog has Fire Blast and Mind Blast. He buys the Sleep and Area-Cone metablasts. He decides to add Area-Cone and Sleep to Mind Blast. He cannot use either with Fire Blast without getting additional schticks. Note that he cannot get Sleep for Fire Blast, as it is not listed as a possible metablast for Fire Blast.
Many metablast schticks can be had for free with some limitations. These only apply to one particular blast schtick; that blast is then limited whenever you use it but your other blasts are not. Having several such "free" metablastson on a single blast is often conterproductive, as you must fulfill all the limitations in order to use it.
It can be very costly to have several blasts and metablasts simultaneously. Most casters get ether a few blasts and several metablasts, or many blasts few or no metablasts, possibly using "free" limitations to get specialized blasts for specific uses.
Exclusive Metablasts
An exclusive metablast has none of the normal effects of that blast, including any other metablasts that would ordinarily apply. In effect, an exclusive metablast is a separate effect that replaces what the blast normally does. Still, it takes it's appearance from the blast it is used with, if it does any damage it is of the same type, and a Damage Imunity that would protect against the blast also protects against the exclusive metablast.
Magic Point Cost
Many metablasts cost Magic points to use. If whatever you are using to blast is abundantly available all around - such as fire in the scorching sun, lightning in a thunderstorm and so on, you don't have to pay the Magic point. This kind of situation must be unusual to count; using air to buffet your foes would only give this bonus in an unusually windy spot, not anywhere outdoors.