Hint: Almost every blog has got free account with feedburner (Now associated with Google)... So it has got RSS subscription button. Right! You need to extract the last word from that RSS URL which is the username of that blog at feedburner. Give that username in the textbox and hit find to calculate subscribers of that blog... Simple!!!
Limitation: This method works for almost every blog, but not for all. Only blogs with feedburner accounts will show their subscribers via this method. If the result page does not show anything that means the method is not allowed for that blog.