Flashing blade as a seal makes me angry

Because..... Idk. I genuinely can’t think of why; It’s absurd

That's a good question, actually.

Maybe because Hone Cavalry/Fliers/Armor(?) are things.

I can only imagine just how annoying it would be to see these extremely fat cavaliers and fliers running around with Flashing Blade when they have such easy access to Goads/Hones.

No thanks.

And no, "fat" was totally not a mistake. I meant to type that.

At the time that Flashing Blade was first available on Mia, Horse Emblem was the dominant team in Arena. Plenty of them were quite fast, like B!Lyn, or could be boosted to really high levels of Spd through Hone Cavalry. Fliers all tend to have really high natural Spd and superior mobility compared to the other movement types, although they were nowhere near as powerful as Horse Emblem because of how rare Hone Fliers was (and still is), and that Ranged Fliers were still pretty uncommon.

There were also no good ways to buff Spd on Infantry units outside of Hone Spd, and Armors were mostly slow. (I pretty sure this was a time before Bold Fighter and Vengeful Fighter were a thing.) I guess that IS saw it as too oppressive to allow Horse units to get faster cooldown charges through Spd, and didn't want to turn Flier Emblem into the oppressive force that Horse Emblem was. Though, I do agree that the restriction is still quite stupid.

Honestly horses with it may not be so bad since they already can use Heavy, but what do I know I'm just a guy.
Anyways really fun units to use it with would be Legault, Athena, Neph, Hana, LonQu, really just anyone with high speed but middling attack (except on horseback rip Titania once more)

I wanted to give Cordelia the Flashing Blade seal too. Only difference is, that in my case i wanted to give the seal to the regular cordelia.

Its really weird why that isnt possible, at first i swapped her heavy blade i gave her first with a unit which i gave flashing blade seal, but then all of a sudden during the battles cordelia didnt have the seal. At that point i started to realize that the seal isnt available for her, which would never come to my mind actually.

Ok, i can understand the explanations of the others here very good, as Cavalry or Flier Unit, you can get high speed much easier with all the hone and goad buffs. Although, tactic skills can do the same but are still hard to get, especially the spd tactic one.

Which is even more funny is, that Caeda and Claire can have flashing blade thanks to their refinerys. Their Eff+ gives them flashing blade, so those two seem to be the only exception.

Up to now, i just gave my cordelia heavy Blade instead which works pretty good too.

I dont know your current build, but maybe you can use heavy blade as an alternative as well?