Removing a darkened caption with text on it in an image
Does this algorithm even make sense. 


Your caption box is actually pretty wide – the algorithm that you've linked to would need to replace the complete content by its neighboring pixels. That won't look right – it will look as if someone "stretched" the guitar there. 

So if you ask me, no, this is not the right approach here.


  Now what I noticed is that the caption is not fully opaque.


I was confused for a while until I realized that with "caption" you mean the box, and I mean the text. So I'll refer to the box to avoid further confusion.

So, yes, that would be what I'd look at here. The image just seems to be darkened in the box area – what about you just take this part of the image, and reverse that darkening, by increasing the brightness? I'd assume that degree of darkening is static – so a bit of trying out in e.g. GIMP you should be able to figure this out; I tried this, and  got this:



as you can see, it's slightly blueish- although the brightness seems to match. That might either be because whatever draws the box isn't actually reducing the brightness only, but somehow also changes the hue of the image in the region, but it might very well also be an artifact of JPEG compression; notice that your picture has a terrible quality to begin with. Or this is a color space conversion effect.