Blotting paper is simply an absorbent piece of paper used to soak up excess ink or oil. 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, or clean nibs.
Fold a paper towel in half and blot the stain gently to absorb. Some paper towels fit nicely into your desktop blotter and are also absorbent enough to 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. Blot gently, as wiping may tear the napkin.
If you are really in a pinch, toilet paper works 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 works for some blotter paper substitutes, but does not absorb oil as well as paper. Also, use an old towel for ink to avoid staining a favorite towel.