Thanks for explaining! I didn't know this, but from the context I assumed something along the lines of "ah, youth"/"to be young" etc. Nice to hear I assumed correctly.
Yesss, also it's important to mind the context, since his brother was talking fondly of him the "blue" probably means "youthful" but in some situations one can also use it in a negative way that'd be more like "naive"
I often read and hear Japanese people equates blue with youth, youthfulness, or something similar to that.
Maybe the proper translation somewhere along the line of "how youthful" or something similar.