Blotting paper is simply an absorbent piece of paper used to soak up excess ink or oil. Therefore, you can substitute most any absorbent paper for blotting paper in a pinch, whether you are using it on your face to reduce shine or at your writing desk to remove ink.
Fold a paper towel in half and blot the stain gently to absorb. Some paper towels will fit nicely into your desktop blotter and are also generally highly absorbent and can be used multiple times.
Paper napkins can be used to absorb oil or ink, though they may be too thin when unfolded, depending on the quality. Be sure to blot gently, as wiping may tear the napkin.
If you are really in a pinch, toilet paper will work to absorb oils and inks. If it is high-quality, you can use a single layer. Otherwise, you may need to fold the toilet paper into a double or even triple layer to avoid tearing.
A terrycloth towel will work for some blotter paper substitutes, but will not absorb oil from your face as well as paper. Also, use an old towel for ink to avoid staining a favourite towel.