If a model with mobile is sitting in blocking terrain can it fire out of said terrain or can it merely be shot at?

Both. Since you draw LOS from corners, a mobile figure within blocking terrain would be able to draw LOS normally, but the below rule ensures the same mobile figure within blocking terrain can still be attacked.

"Mobile", RRG, Page 19:

If a figure with mobile occupies a space containing blocking terrain, line of sight can be traced to that figure, spaces can be counted to that figure, and adjacent figures can attack that figure.