I've used mini blinds for over 10 years in my gardens. The first thing I will say "Sharpies will fade out usually before a year has passed. I do not use them for anything other than writing on my seed packs.
Paint pens work real well but can be a tad messy.(on mini blinds). I'm a big supporter of pencils for mini blinds. I've used the regular old yellow #2 and have masses of them in the greenhouse. But, my favorite is a #9 lead mechanical pencil. These came from Staples and the ones I'm using were bought about 6 years ago. I love them. #9 lead is hard to break and bigger, thus you get a bolder line. I will list the name brand for Staples because the #9 pencil is hard to find at Walmart or most other places. Of course, you can use #5 or #7 but I find they break so easily and the good old wood pencil must be sharpened.
Speaking of sharpening a wood pencil, If you find yourself outside and need your pencil sharpened and don't have a sharpener handy, find a brick or anything concrete and rub the end of the pencil at the same angle that it was previously sharpened against the brick or concrete. It will put a very usuable point on your pencil.
Now, here's the info on the pencil I really like:
Pentel Twist-Erase® III Automatic Pencils .9mm, Assorted Colors, 2 Pack
Item: 474462 Model: QE519BP2
You will also need to buy some extra lead and be sure to get #9. You can't find this lead just anywhere. And if you're like me, put it somewhere you can find it!!!
