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You can use a gradient to gradually blur a background.
Be sure to check off as you go along.
1) Preserve your original file.
If you haven't already done so, go to Preserve Your Original File.
If you haven't already done so, go to Make a Background Copy.
1) Create a Levels adjustment layer.
Click the Create adjustment layer icon, the half-dark, half-bright, circle at the top of the Layers palette, and select Levels.
The Levels window will open.
You'll only be using this layer for its mask, not for any exposure or contrast changes.
Click the black arrow, to the right of the Gradient Picker in the options bar, to open the Gradient Picker window.
Select the Foreground to Transparent gradient.
Click elsewhere on the screen to close the Gradient Picker window.
In the options bar, make sure Linear Gradient is selected, the icon on the left.
Also, in the options bar, make sure the Mode is Normal, Opacity is 100%, Reverse is not selected, and Dither and Transparency are selected.
4) Make sure the foreground color is set to black.
Press x to cycle between the foreground and background colors.
If the colors are not black and white, press d.
5) Make sure the Levels adjustment layer is active (highlighted).
6) Click on the photograph where you want the blur to begin, hold, and drag a line to the top of the photograph.
Here, the gradient was drawn at a slight angle to avoid blurring the buildings on the right side of the photograph.
The gradient will appear in the mask thumbnail of the Levels adjustment layer.
White reveals; black blocks.
Where the mask is black, the blur will be concealed.
As the mask fades, more and more of the blur will be revealed.
7) Press and hold Ctrl, and click the mask thumbnail in the Levels adjustment layer.
A selection will appear on your photograph.
The marching ants display is smaller than the actual selection.
The marching ants display only shows selected areas that have brightness values of 128, 50% gray, or brighter.
The areas in a selection with brightness values darker than 128 are not shown.
So, as happens in photography so often, what you see is not what you get.
The selection of the gradient is larger than what the marching ants display shows.
8) Make sure the Background copy layer is active (highlighted).
9) Go to Filter > Blur > Gaussian Blur.
3.5 pixels was used here.
The background will become blurred.
10) Press Esc, or Ctrl + d, to deselect the selection.
Compare the two versions by deselecting the Eye icon of the Background Copy layer.
Original
Blurred with Gradient
My apologies to Monsieur Daguerre.
Go to Blur the Background.
If you haven't already done so, go to Saving Files.