Your links are wrong.
Try
{{ URL::to( 'blog/' . $previous->id ) }}
and
{{ URL::to( 'blog/' . $next->id ) }}
Then, in your controller, you do
$next = Post::where('id', '>', $post->id)->first();
and you get the previous post in a similar way.
By the way, you should also check that there is a next or previous record. You might be viewing the first or last one.