There's simply too many variable in your question to answer it adequately. Blunderbuss refers to a type of firearm, and they could vary dramatically. The damage you'd receive from it would depend upon the caliber of the particular weapon, the amount of powder being used, the type of shot that was loaded, which could be anything from regular ball, shot (akin to what modern shotguns fire), nails or even glass. Additionally the range in question would matter ("close range" isn't an exact measurement), as would the location on your body where it hits you.

So depending upon all those factors, the damage it causes could range from negligible to catastrophic. Fun fact: the blunderbuss' famous flared muzzle was NOT meant to cause the ammunition to spread out, but rather to ease loading.