I've seen references to the Blue Pill before but I've never really researched it. Wikipedia gives a brief explanation of the pill at this link You can also read on the Blue Mass another name for this medicine at wiki as well. Check out the toxicity in that article as well. As I read up on this medication I see no healthy use for it, however, if you're trying to kill someone heavy metal poisoning might do the trick.
Below is advice concerning the use of the Blue Pill during the 19th Century not today.
Blue Pill—Exerts a decided influence over the liver, by increasing the flow of bile. It is deservedly esteemed, and may be usefully combined with rhubarb, colocynth, and other laxatives, in cases where the bowels are torpid. It is mild, and may occasionally be given to children when intended as a purgative. In affections of the liver, such as torpidity or want of proper action in that organ, three or five grains of Blue Pill, with the same proportion of Cathartic Extract, may be taken with much advantage once or twice a week at bed-time, followed by; a Seidlitz Draught in the morning.
Source: A Family Medicine Directory ©1845
The blue pill as described in the Dictionary of Daily Wants ©1866 says:
Blue Pill--One of the most useful, safe and convenient preparations of mercury. Its use for general purposes has almost superseded calomel, and has this great advantage over all other forms of mercury, that it may be taken with comparative impunity, and employed by the non-professional person with almost absolute safety; an overdoes having only the effect of a purgative, passing out of the system by excessive action it superinduces.
And the 1886 Family Physician says:
Blue Pill or Mercurial Pill, like the last preparation, contains mercury in a state of minute subdivision. This is, undoubtedly, the oldest, as well as the most popular form of mercurial pill, and is said to have been first introduced by Francis the First, King of France.
It is made by rubbing together mercury, confection of roses, and powdered liquorice root. Steam power is now employed for this purpose, a considerable advantage, as the efficacy of the pill depends on the extent to which the mercury is. subdivided. A blue pill may be given either for its purgative properties, or for itseffects on the general system. The practice of taking a blue pill at night, followed by a black draught in the morning, is a very popular form of obtaining a purge. It must never be forgotten that "blue pill" is a mercurial preparation, and that its constant use may possibly cause salivation, and the other symptoms which arise from the over-action of mercury.
The constitutional symptoms of mercury may be induced by taking blue pill with opium (Pr. 62); the opium neutralises the purgative properties of the mercury.
For reasons that I am unaware of Mercury had it's place in 19th Century medicine, this throws me since I grew up when people didn't want to buy canned tunafish because the mercury levels were high. But this source gives a brief explanation on Mercury in Medicine that might help you understand why they used it.