"The Adventure of the Two Women" is a Sherlock Holmes crime story by Adrian Conan Doyle. The story was published in the 1954 collection, The Exploits of Sherlock Holmes.
Holmes is called upon to save the widow of a distinguished family from shame. Her noble husband, Henry Gladsdale, the late Duke of Carringford served his country well, but now the Duchess Carringford and her betrothed daughter, Lady Mary Gladsdale, are facing ruin. A blackmailer threatens to expose a prior marriage by Henry Gladsdale that would nullify his marriage to the current Duchess. The price for silence is Gladsdale's private papers, which undoubtedly contain information valuable to foreign powers.
Holmes must fend off the threats of a thug, discover the identity of the blackmailer, and commit a crime himself before he resolves the extortion plot.
"They appear to be perfectly in order, and the signature is the same in both cases."
"Quite so. But the ink?"
"There is a shade of blue in it. Yes, certainly it is ordinary blue–black indigo ink. What then?"
"Every word in both documents is written in black ink, with the exception of the bridegroom's name and signature. Does not this strike you as curious?"