Web is exclusive; your blast does no immediate damage and has none of the normal effect. Instead, you set up a sticky web with a diameter equal to your Magic in meters, that will hinder the movement of anyone inside it. The web is clearly noticeable, but transparent, and lasts until the end of the Sequence. The puddle has no game effect after the end of the sequence, but is still there and still foul until cleared away. If you spend a Magic point it lasts until dispersed (at least a few minutes). The effect need not looks like a web; a sticky patch of slime or even a hindering field of nebolous energy works just as well.
Anyone in web must spend one extra shot to take any action, and must make an Agility task check with your Magic as the difficulty in order to move, and even then they move at half rate in the web.
Damage Immunity to Water or Earth rather than Blast protects against this. A creature whose Strength is greater than your action value can ignore the web, as can anyone with Wallcrawler, Webwalker or similar abilities.
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.
If you can only use the blast to create a web, and cannot use it for plain damage, this metablast schtick is free.