By default, the Blob Brush, will consume and join with any object of the same color Fill (with no Stroke) as you click and drag over the object making the resulting object one big vector shape. These eating habits include both selected and unselected objects. Is there any way to make the Blob only combine with selected objects? Sure! Just double-click on the Blob tool in the Tools panel to open its Option window. Check “Selection Limits Merge” and press OK to apply. Now the Blob will only merge with the selected objects that it comes in contact with.

 On behalf of layers magazine.