The Blob brushes can be used only on Blob layers and they can never be used on the background layer.
1. Click on the mini bar to create a new layer.
2. Right-click on the new layer and select "Blob Layer (Alpha Smooth Lum4)".
Alpha Smooth Lum4 is the mix mode set by default for this layer.
There are multiple Alpha Smooth modes (Alpha Smooth Lum to Alpha Smooth Lum 5) that can be found in the mix mode drop-down menu on the Layers panel. Any of them can be used when doing blob modeling.