你需要做的，本质上是为Image.paste()创建一个蒙版，只粘贴模糊图像中位于圆角矩形内的部分。

import PIL
from PIL import Image
from PIL import ImageFilter
from PIL import ImageDraw

# when using an image as mask only the alpha channel is important
solid_fill =  (50,50,50,255) 

def create_rounded_rectangle_mask(rectangle, radius):
    # create mask image. all pixels set to translucent
    i = Image.new("RGBA",rectangle.size,(0,0,0,0))

    # create corner
    corner = Image.new('RGBA', (radius, radius), (0, 0, 0, 0))
    draw = ImageDraw.Draw(corner)
    # added the fill = .. you only drew a line, no fill
    draw.pieslice((0, 0, radius * 2, radius * 2), 180, 270, fill = solid_fill)

    # max_x, max_y
    mx,my = rectangle.size

    # paste corner rotated as needed
    # use corners alpha channel as mask

    i.paste(corner, (0, 0), corner)
    i.paste(corner.rotate(90), (0, my - radius),corner.rotate(90))
    i.paste(corner.rotate(180), (mx - radius,   my - radius),corner.rotate(180))
    i.paste(corner.rotate(270), (mx - radius, 0),corner.rotate(270))

    # draw both inner rects
    draw = ImageDraw.Draw(i)
    draw.rectangle( [(radius,0),(mx-radius,my)],fill=solid_fill)
    draw.rectangle( [(0,radius),(mx,my-radius)],fill=solid_fill)

    return i

面罩：

将蒙版应用到图像：

img = Image.open('pic.jpg')

x, y = 300, 160 
radius = 75

cropped_img = img.crop((x, y, 600, 600))

# the filter removes the alpha, you need to add it again by converting to RGBA
blurred_img = cropped_img.filter(ImageFilter.GaussianBlur(20),).convert("RGBA")

# paste blurred, uses alphachannel of create_rounded_rectangle_mask() as mask 
# only those parts of the mask that have a non-zero alpha gets pasted
img.paste(blurred_img, (x, y), create_rounded_rectangle_mask(cropped_img,radius))

img.save('new2.png')
img.show()

我改变了一些维度和路径。你的代码缺少导入，我把它修改成minimal verifyable complete example。