OpenGL : Blending & feedback effect

I’m struggling on a simple project, as an example/sandbox, I’m rendering a small oscillating rectangle on my output. I’m not using glclearcolor() but instead, on every frame I draw a black rectangle before anything else, blending with glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

My goal is to see as I play with the alpha of this black rectangle feedback of previous frames, slowly fading, some kind of trail, and it’s more or less working.

My main problem though, the trail never really disappears, and the longer I try to get the trail, the worse it gets. Also, I have to quite crank the alpha before seeing any trail, I don’t really understand why.

enter image description here