This tool is one of the best tools if you want to create a sketch within Illustrator. You can really draw and add to an existing shape like you would if you were drawing with an ink pen. the best part about this, is that with a drawing tablet, you can very closely simulate freehand drawing and sketching, but with this, erasing and cleaning up your artwork is much easier. With enough practice, you could even eliminate the step of sketching something out on paper, scanning it in, and re-tracing your own artwork. This can be a bothersome and tedious procedure.
The blob brush is located under the brush tool on your tools panel. You can open up the brushes panel and chose what type of brush that you would like to work with. I keep this panel open, because you can easily choose between so good, basic sized defaults that you would use when sketching your artwork. I created this number 4 by using the different presets, which are different thicknesses, and when I did this, it automatically adds to the shape. It doesn’t make individual shapes and pieces. It automatically merges your ink into one form. If you want a separate form, then you need to change the color. I chose red to give you an example. It makes its own separate shape.
The middle brush at the top is a flat brush, which means that it will change its thickness depending on the direction that you pull the brush. You can create your own brushes at any time, with any thickness and any behaviors, even pen pressure. I made the thicknesses random, and I was able to achieve a look of ribbon with this new brush. This would be great if you wanted to create an abstract sketch.
If you take your own reference photos, then you could use the blob brush to trace your image, and then give it a stylized look, and it would be one simple vector. The best part about doing a sketch with Illustrator is that it is vector based, and would be infinitely scalable.