Just curious, why are there so many blue haired swordsmen?
There honestly aren't that many. It's just quite a few tend to be the main character of their game. To quote myself from before
Lets see for a grand ol comparison all the main characters in the series
FE1/3/11/12
Marth, Blue hair, sword user
FE2
Alm, Blue hair, sword user
Cecilia, Red hair, Magic/Sword user
FE4
Sigurd, Blue hair, mounted Sword/Lance promoted character
Seilph his son, Blue hair, mounted Sword/Lance but started as a lord with just a sword
FE5
Leif, Brown hair, Sword user, but in FE4 promoted to a mounted unit who could use literally everything but Dark Magic
FE6
Roy, Red Hair, Sword user
FE7
Eliwood, Red hair, Sword user then lance mounted
Hector, Blue hair, Axes got Swords on promotion
Lyn, Green hair, Katana styled sword use. Gets Bows on promotion
FE8
Ephraim, Blue hair, Lances
Eirika, Blue hair, Swords
FE9/10
Ike, Blue hair, Swords. In his sequel late game Swords/Axes
Micaiah, Silver Haired, Light Magic/Staves
Sothe (He semi counts) Green hair, Knives
FE13
Chrom, Blue haired Swords promotes to be able to use lances. Has reclass options that include mounts, bows, Axes
Lucina same as above except more weapon chances due to more reclass options
Robin, Magic/Swords any hair color, default white. Endless reclass options