As far as art equipment goes the humble make-up sponge is a must have!
For starters they are cheap and while they will not last forever… if you are careful then you can use them more than once.
You can also have specific sponges for specific colors. I use them while working with graphite, charcoal and pastel.
Although they are small, I find then quite versatile. There are three surfaces, two flat surfaces that are particularly good at blending, but also produce a broad enough mark that is useful for lightly marking in the direction of fur.
The third surface is the edge that joins the two sides and it creates a narrower mark. It is particularly useful for loading up with charcoal or graphite and creating smaller darker areas.
You just need to experiment!
For me the sponges have made the rendering of the fur on this cheetah much easier than it would otherwise have been!
Still a long way to go before this big cat is finished but good progress has been made!