Blá blá blá is a Portuguese onomatopoeia, similar to the English "blah blah blah". It's used to describe meaningless talk or empty words.
In this song, Solange Almeida uses it to express her frustration with a former lover's insincere apologies and empty promises. She's tired of hearing their "blá blá blá" and wants them to leave her in peace, making it a very relatable and impactful phrase in the context of a breakup.
This is a breakup anthem sung from the winning side of it. Solange Almeida and Márcia Fellipe trade verses about a man who used to swear things like "Que por mim você morre de amor" [That you die of love for me], and now they are done buying it. "Não adianta me pedir perdão" [It is no use asking me for forgiveness], one of them sings, because the apology comes too late: "Se não cuidou, só maltratou" [If you did not take care of me, you only mistreated me].
The title line does the real work. "Não quero mais ouvir esse blá, blá, blá" [I do not want to hear this blah, blah, blah anymore] turns his excuses into noise, three empty syllables standing in for an actual reason. By the second verse the balance has flipped completely. "Agora esse jogo virou" [Now this game has turned], she sings, "quem não te quer agora sou eu" [now I am the one who does not want you]. It is a forró duet built for a crowd to shout back at, and every "blá, blá, blá" the audience yells along is proof the song already won its argument.