Conforming to the interface

Config/thumbnails.php

@@ -6,6 +6,8 @@ return [
             'width' => '200',
             'height' => '250'
         ],
-        'blur' => '15'
+        'blur' => [
+            'amount' => '15'
+        ]
     ]
 ];

Image/Intervention/Manipulations/Blur.php

@@ -12,6 +12,6 @@ class Blur implements ImageHandlerInterface
      */
     public function handle($image, $options)
     {
-        return $image->blur((int)$options);
+        return $image->blur($options['amount']);
     }
 }