When I saw blood sausage, the first thing I thought of was English bitter. Don't get me wrong, I love German beers, but there's something so quintecentially British about blood sausages and other ofal dishes, that English beer seems more appropriate. If you can find it, buy a four-pack of Abbot Ale. Failing that, try Fuller's ESB or Old Speckled Hen.
While the British make wide use of offal, they are hardly the quintessential example. Whenever I see a German butcher butcher shop, I know that they will likely sell blood sausages and, in the Yorkville neighborhood of NYC, there is Hungarian butcher that sells both blood and liver sausage. The french sausage, boudin, is also a blood sausage. I fell certain there are other European examples.
Oh, I do like me some blood and liver sausage.
Since we are dealing with German food here, German style beers are in order. Not sure how spicy these will be as there is great variability among blood and liver sausage styles, so I would approach it in one of two ways.
If the sausages are spicy, a nice hoppy pilsner would be a good match. If they are more savory, I would seek out a good schwarzbier. If you have access to both styles, they would make an interesting contrast to see how the interplay between a hop forward versus a malt forward style interplays with the food.