To me the most mild blade is the Derby. However, some folks do not like them because they are not the sharpest. Other blades that are smooth - to me - are the Lord blades and Personna Reds.
For me the Personna Med or Lab blades are the best combination of sharpness and smoothness.
Gillette Nacet Platinums. Not a blade that gets a lot of attention, mostly because it's not a jaw-dropper of a blade. It's a decent shaver and will get the job done without setting your face ablaze.
I started with Derby and they were fine. But, if I was only beginning today I think I would go with Astra SP. I love Personna Blue Labs, but they are not as mild as the Astras--YMMV
Timor, sharp but silky smooth.
And Dorco ST-301's work too for me, excellent newbie blade I think. Not too sharp but very forgiving. I learned my skills mostly on these blades.
Personna Reds and Timors are my favorite mild blades. 7 o'clock Blacks are very sharp, but also forgiving for me, as are Med Preps. Silver Blues are mild and forgiving, good blade.
I guess I still don't know what mild is. Maybe I don't know what is not mild either. My opinion on razor blades is the same as knives: Dull is dangerous. But all razor blades I'm aware of so far at least start out sharp.
But Derby blades do have a reputation for being forgiving, whatever that means. I happen to think they are ok. I think the Astra SP blades are also ok. I have also used the Wilkinson Sword blades from Walmart and they are fine too.
I have also used a store brand which I understand is made by American Safety Razor. They are not as smooth as the three other brands I mentioned (and also over priced).
With what I know now, I would recommend the Astra SP. The price is right and they are good blades.
Derby is definitely a mild blade. I like it a lot as it can get the job done without worrying about nicks. As I just recently started back to DE shaving, I am also trying other blades such as Personna, Astra, and Feather to get an idea as to which works best for me.