Okay, this will probably seem a bit sacrilegious to those who do not want to mark a print in any way, so I apologize.

First, a preface to avoid responses telling me not to do it (though those prefaces rarely seem to work):
I am going to write on the back of my 4x6 prints; always have. These are just snapshots in an album. I do have the negatives in case anything bad happens, so can always print them again. Also, I usually order double-prints, so have "backups" that are not written on. Anything to be framed, presented, or otherwise done nicely is not written on.
Yes, I have thumbnail lists to identify some rolls. However, labeling the prints themselves will make them identifiable if they are separated from everything else. These are not "art" or "craft" photos - they are simply my memories, just like all of the 70-year-old (or more) photos my grandfather had of people and places we wish we could now identify (most of which fell off the pages of lost albums).

Again, I do this and will continue.

What fine or ultra-fine tipped writing utensil would you suggest? I'd like brand/model and place to purchase if you can.
I don't want it to bleed through print for at least a couple decades, nor "crease" it. These are just wet prints from minilabs, and I put them in sleeved album pages.

Now, I have done this since I was a kid. I write some description (usually people's names or places) and the date on the edge of the back of the photo. I'd used whatever was available (marker, ballpoint) until college, then tried to stick with felt-tips or really nice gel pens (read, very easy flow).

I currently have two problems that have led me to ask this question.
1) It has become difficult to find good non-repro blue pens/markers.
2) RA-4 paper keeps getting thinner.

I don't care if I can see the writing with a back light as that is not how they are viewed.
I had searched online photography stores to no avail - perhaps I merely missed them.
Recently I tried a couple from pens.com. Close enough to the blue I want. However, one is a ball-point, and can leave pressure creases if pressed hard enough to write, the other is a gel that does not flow well enough, and can cause the same issue if I am not very careful. These pens are otherwise decent, so have been re-purposed.

I'm testing ultra-fine tipped ("precision") Sharpies, and they seem to not bleed through, but may be dark enough to see from the front. Also, I'm testing on prints from a 120 roll so I only have to reprint 10 photos if something bad happens immediately.