SGS2 screen gets more and more dark blue, possibly after overheated charging My phone screen is gradually becoming dark blue and then black--see photo. Started with a single light blueish edge, and growing during the last hours to what can be seen on photo.



The only serious reason that I could think of is that the phone has recently overheated while charging: it stayed for several hours under the pillow, and it was very hot when I took it away from there. Another guess is a recent drop on the floor, but in its 2-years lifetime it dropped dozens of time, frequently much harder than today.

What could be the reason, and how it can be fixed, ideally DIY or at all?

It's Samsung Galaxy S II GT-I9100.
Looks like a damaged LCD that's slowly "leaking out."  Overheating could definitely be the culprit, as could the drop. Although damage from the drop is usually accompanied with a visible crack.  Most likely it was the combination of the two.  The fact that the phone booted up and is (sort-of) functional leads me to believe that replacing the LCD assembly would fix it.

There are plenty of replacements to be had on eBay for up to USD$100.  I would go for the LDC+digitizer combination, even though the glass itself looks intact. There are simply too many issues with properly separating LCD from the glass that could cause you to break both and spend even more money.  It is definitely doable on your own, provided you have some basic tools.  There are videos on YouTube with great walkthoughs to help you along the way: probably most detailed (even though it's not on GT-I9100, but on other modification of SGS2); some others specifically on GT-I9100. A guide at iFixIt can be also helpful.