Possible Clues:

• Hides half of Jacob's smokes
• Doesn't show up the stink created by individual getting top-secret treatments
• Makes less visible or unclear
• Old salts must welcome British Standard for shrouds
• Conceals sources of organic black sausages and kippers
• Casts a shadow over what doctor does after oxygen blood serum extremes
• Conceals old boy's remedies
• Covers, darkens
• Makes it hard to find the remedies at last