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.
First of all, we have to apply the foundation creme.
If you have very dark rings under your eyes,
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!