What the heck is a sublimation blowout sheet? It’s that sheet of teflon that came with your heat press and it has two purposes in the sublimation process. The first is to protect the surface of the substrate from any dirt, grease, stray ink, or whatever on the heat press during pressing. The second is to prevent any stray ink on the transfer paper or substrate from migrating onto the pressing surfaces of the heat press.
While most people use the teflon sheet supplied with the press, eventually it can become soiled in which case it loses its effectiveness. A great alternative is to use blank newsprint, butcher paper, or silicone sheets. It’s cheap and quite absorbent, but also a one-time-use product, so discard after every pressing.
By the way, if you are pressing both sides of an apparel product like a bag or shirt, you will want to add a second blowout sheet underneath when you flip it over to do the second side. You may also want to add a third one inside the item to prevent any unwanted transfer of ink from the previously pressed side over to the side bring pressed.