Why does 你可以再用一点遮瑕的东西 use 东西?
Why not use 东西？Well, in English you also would probably not say, 'apply the foundation creme thingy.' 'apply the foundation creme' suffices. The writer knew it was a liquid,液.

"再用一点遮瑕的东西。" The writer is not sure what exactly this is: creme, powder, liquid, emulsion, so he/she said 'thingy, 东西'

Concealing blemishes is a science. What is best is a quandry:


  BLEMISH-HIDING TIP #3: PICK YOUR FLESH-TONED CONCEALER WISELY
  
  When it comes to disguising imperfections such as blemishes on your
  face, a creamy concealer is always a good idea. A powder can actually
  give your skin a cakey look and draw more attention to the thing you
  wanted to hide in the first place. No thanks! After applying your
  color correcting or highlighting concealer, try the L’Oréal Paris
  Infallible Total Cover Concealing and Contour Kit, which comes with
  four flesh-toned shades. Apply the closest match to your skin tone
  with your makeup blender so that you can cover your pimple from angles
  that you wouldn’t be able to reach with your finger. And don’t rub the
  concealer on—be gentle by lightly dabbing and rolling the makeup
  blender over your skin.


首先，我们要涂粉底液。
First of all, we have to apply the foundation creme.

如果你的黑眼圈很严重，你可以再用一点遮瑕的东西。
If you have very dark rings under your eyes,

你可以再用一点遮瑕的东西。
you can use a little blemish mask to cover them up.

Actually, I think the blemish mask should come first! It's hard to look beeootiiful!