It isnt always necassary to block 9the wtting and drying) a swatch, particularly for somthing like a pillow ase where fit and size aren't uber important. I often do swatches which i don't block that i don't cut the yarn for and can then unravel and re-use.
If you do unravel a blocked swatch, you can re-knit with it, but you might find that it has a lot of kinks in it and is difficult to work with without giving it a chance to relax, which can be done in a few ways such as by hanging it straight with a small weight on, wrapping it loosely back into the skein or dampening and drying straight and flat (re-blocking it).
I am Tarriknits on Ravelry
This is my not necassarily knitting related blog