Blindada translates to "shielded" or "armored." It's a strong and evocative word that isn't commonly found in everyday song lyrics, making it stand out.
In the context of the song, the singer uses it to express a selfless desire to protect his loved one from harm. He sings, "Pero que tu alma se quede blindada" (But may your soul remain shielded), indicating his willingness to take on pain himself to ensure her emotional safety. This powerful imagery of protection makes it a memorable and impactful word.
Carín León spends this whole song arguing with himself, and losing. He opens with the verdict already in: "Hay historias que terminan cuando empiezan" (there are stories that end when they begin), and he has decided this is one of them before anything has actually gone wrong. The reason is not that he stopped caring. It is the opposite. "Estoy sintiendo mucho y me da miedo" (I'm feeling a lot and it scares me) is the admission the rest of the song is built on, and the exit plan arrives in the very next breath: "Prefiero marcharme ahora que puedo" (I'd rather leave while I still can), specifically "Antes del primer 'te quiero'" (before the first 'I love you').
The chorus is the uncomfortable part, because he knows exactly what he is doing and does it anyway. "Cuando alguien me importa, huyo" (when someone matters to me, I run) is said as a fact about himself rather than a choice, and "Lo que toco lo destruyo" (what I touch I destroy) is clearly an excuse he has used before. What saves it from pure self-pity is the trade he offers: "Yo puedo joder mi corazón, pero no el tuyo" (I can wreck my own heart, but not yours). He wants credit for leaving. The song never says whether he has earned it, and by the last minute he has run out of sentences and is just repeating the title.