You might be best served by having someone that works with brick take a look at what's stained it. You can try contacting the Local Historical Society for references. This way, you'll be sure of what you have. I wouldn't sand-blast or use anything real caustic w/o knowing what it is. You may damage the brick by either staining it or something.
No one on this Forum (that I've seen), is in favor of painting or staining brick. Once that's been done, you've turned a maintenance-free material into one that now requires maintenance. Search the Forum page for brick and keep searching until you find ALL the past Posts that talk about painting brick (or staining it.) It may be just what you need to hear.