It's happened to me once, and what I figured out was after I finished my injection and leaving the needle in for about 7-8 seconds, I realized I was pushing down way to hard on the end cap of the pen because I was wanting to make sure I got my entire dose. Well before removing from my skin, I would remove my thumb from the end cap and by doing that, it created suction and a drop of blood got in the cartridge.

Whats your research telling you is the best option for manual injection pens?

I usually just write on a label a concentration in mcg/units. For example, if I reconstitute 10mg (10,000mcg) in 2 ml (2,000u) I put "x50". It is very useful, for example if I need 500mcg (0.5mg) I use 10 units since 10x50=500.