Best way to fill space when making a square photo into a wide one

[prodsix]

I'm new to paint.net I was wondering what is the best way to blend new space in a photo to make it perfect

 

in the photo below it's an album cover and I'm trying to use it for something, but want it to be a 1920x1080 photo, I'm not sure what the best way to make the whole sky fill up the black bars too so that it blends perfectly, I tried using some tools but you can see where the colors clash where it's lighter and darker, big rookie here, have no idea how to do this

 

  I

[Pixey]

Hello @prodsix and welcome to the forum  

 

1.  I would remove the black edges, to leave a transparent space each side.

2.  Use the Rectangle Select to 'grab' the side of the blue.

3.  Then slowly drag it out.

 

To get this, and repeat on the other side:

 

[Tactilis]

Follow @Pixey's suggestion and then Effects | Blurs | Gaussian Blur in the new blue area to smooth out the slight horizontal streakiness that you get on the right hand side.

[Pixey]

2 hours ago, Tactilis said:

Gaussian Blur in the new blue area to smooth out the slight horizontal streakiness that you get on the right hand side.

 

Depending on where one begins the 'box' with the rectangle, will help to minimize any streaks.  I think on the left side, the start point needs to be narrower than the right side.

 

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