Many, many, many years ago (I'm old)
.. I was taught to use chalk or the small slivers of leftover bars of soap to mark the "wrong" side of fabric. As for using several shades of black in a project.. that's not always the look I want.
Example:
[ATTACH=CONFIG]476873[/ATTACH]
[ATTACH=CONFIG]476874[/ATTACH]
I can see using different blacks in a scrapy quilt. Just wasn't the look I was going for in these two.