A sheer gel blush best for fair to light skin applied over either bare skin or liquid/cream foundation but under any powder product. It is sold as a low maintenance blush but unless you have perfect skin it takes some work. Applied with fingers it is too patchy, results are much better buffed in with a plushy synthetic blush brush. It is sheer so imperfections show through and are accentated (blemishes, age spots, dry patches).
I'd say its either for young perfect skin, or, for people who are able to get it looking that way with a routine of exfoliate, moisturise, conceal plus foundation. Thats too over the top for me to make a single blush work I didn't really like the product. I know thats the deal with most Gel Blushes but YSL is pricey so I expected more. Plus I've used Revlon's gel blush in the past and I thought that was better.