Gaussian Blur lets you apply an even blur to the current layer.
Prepare 2 illustrations, and add blur to one of them and Darken blend mode to create a soft-looking illustration.
Open ①the Layer window, and ②duplicate a layer, and select the duplicated layer.
Open ①the blend mode and select ②Darken.
From ①the Tool Selection window, select ②Filter.
Select ①Gaussian Blur.
Drag ①the slider left and right to make adjustments.