Not because I'm poor and ugly, but because I just felt like it
I'm wearing a black shirt
Or: I wear the black shirt. Black is likely a reference to funerals, where people wear black
Today my love is mourning
Estar de luto = to be visibly mourning (a death)
Today I have a sorrow in my soul
And it's because of your spell
Or: And it's the fault of your curse
Today I know that you don't love me anymore
And that's what hurts me most
Because I'm wearing a black shirt
And a sorrow that hurts me