The best thing to use for long lasting lettering on these vinyl labels is a grease pencil, also known as a china marker. These can be purchased relatively inexpensively at office supply and craft stores.
You can also write on them with permanent markers or pencils, but the writing will weather away after awhile. Carpenter's pencil writing lasts a good while, and they are very cheap at home centers or hardware stores.
If you want a permanent plant marker, you can type the name on a sticky label, put it onto the mini blind label, and paint with clear varnish or polyurethane. For longer lasting permanent marker writing on a label, simply paint with two coats of clear nail hardener.
Mini blind labels can even have the writing scrubbed off for reuse. For permanent marker, alcohol will remove any leftover writing.
You can see how recycling mini blinds into weatherproof plant labels is a cheap way to make sure you always know which plant is which. Watch your neighborhood roadsides on garbage day, so you can save money by making your own free plant labels.
References: This article was written from the author's own knowledge and experience.