Using some bilinear or bicubic interpolation when enlarging shouldn't be called blurring, even though the result looks kind of blurred, compared to nearest neighbour methods (allegro stretch_blit()). Blurring is what you add to a picture, when you don't change the seize. And when you blur, you lose information, when you just want to smoothen some sharp details. You may use stretch_blit() and then you may blur, but you might not end up in same result.
Take a 1024*768 image (digital photo) and scale it down to 256*192. Then make one copy that you scale back to 1024*768 using some bicubic interpolation. Make another copy using "non blurring" stretch_blit(). The former is definitely more near the original. That's why it is stupid to talk about blurring when upscaling using bicubic interpolation.
If your old photograps are on photo paper, use the best scanner you can get. Hopefully you can turn the whole issue into a downscaling issue. If they are digital images, you have to stick with the stuff on the page Miran linked to.