As indicated by the name, a blanket mold resembles a thin blanket of rubber placed around the model. Rubber blanket molds are generally 1/4″ to 1/2″ thick and are made either by:
1) brushing or spraying flexible mold rubber onto a model and then creating a mold shell around it (the brush-on method generally requires application of multiple layers of rubber)
2) pouring the rubber into a previously constructed mold shell that is situated around the model.
In all cases, construction of a rigid mold shell, also known as a “mother mold”, is required in order to keep these thin molds in the proper shape and position for casting.
Read more about blanket mold basics here: https://www.polytek.com/tutorial/rubber-blanket-molds-basics