[Konzern]

There's several blue haired characters in Fire Emblem, including Marth

His descendants Chrom

And Chrom's daughter, Lucina, who does dress as Marth at one point

Virion

Ike Ike also has a descendant with blue hair, Priam.

You can also be quite literally any of the kids from Awakening or Fates or either avatar (male or female Robin or Corrin.) With the kids their hair color is depending on the father in Awakening and mother in Fates. If you have a blue haired character (like Chrom and Virion) be the father in Awakening, the child gets their hair color. Not to mention if you customize the Avatar to have blue hair, and is the father in Awakening or mother in Fates, the kid will have that shade of blue hair.

Ciel Phantomhive's hair is a dark blue

Lawrence Bluewer also has blue hair

Sophie's hair is a light blue

And the oddball of the ones I can think of off the top of my head, Marie Canker Probably the easiest to do though. If I can think of more, I'll come back.