The Anti-Magic flows within his body now, that's why the sword became black. It honestly doesn't make sense how licht can use the swords considering he definitely has magic in his body, but I'm sure they'll explain that later
Yea, he took the sword which gave Asta a chance to use other people's magic... Looks like Tabata decided to change what Asta could do... >_> Not sure why Licht bothered to replace it though, unless exchange was required since sword came out of the grimoire that now belonged to Asta.
Most likely, it's very simple. It depletes mana and depleted from Licht - since it became shiny in his hands, but turned rusty/black again in Asta's hands when he caught it. Asta has no mana to absorb. Licht could command all the mana from the surroundings along with his own dense elvish mana. So he had too much of it and could spare enough.
That would explain the color and why he can wield it, but what I find the most peculiar, is that he didn't took the other sword. I mean considering how easily he took the Demon Dweller, why didn't he try to take the Demon Slayer as well? If he did, Aster wouldn't have had anything anymore. It seems rather interesting that Licht said this chapter that that grimoire isn't his anymore, so I interpret that as not that didn't want to take the sword, but that he couldn't.
Yea, reading the last two pages of the last chapter and this chapter at a row it seems Licht didn't exchange it but it was knocked out of his hand while trying to deflect Asta's attack, in doing so it absorbed the antimagic, and Asta grabbed it. Since it turned anti magic Licht let them all go for now.
