Links are displayed in blue font.
Links are currently displayed in black/white font.
Compared to normal text, a link is clickable.
A different color would make it stand out better from the rest of the text.
This is also how it is handled in many other messengers.
It’s not only about links being clickable for me. It’s more about having links alongside other text. Links are usually not intended to be read by humans. So it is nice to have an easy way to see where the link ends and where the actual text from the user starts.
The underline is not prominent enough to me.
DeltaTouch already makes links blue and, unsurprisingly, it works well. 
we are about to change that, here for android: change link color by adbenitez · Pull Request #4011 · deltachat/deltachat-android · GitHub - iOS and desktop will follow
underline will stay, to not make things worse for user that cannot differ colors easily.
thanks all for pushing for that, it indeed is an improvement, esp. for longer links as pointed out above