It is not absolutely indicative of the presence of blood, for tincture of guaiacum is coloured blue by milk, saliva, and pus.
"Aids to Forensic Medicine and Toxicology"
W. G. Aitchison Robertson
The most active part of the tuber lies just beneath the skin, as may be shown by pouring some tincture of guaiacum over the cut surface of a Potato, when a ring of blue forms close to the skin, and is darkest there while extending over the whole cut surface.
"Herbal Simples Approved for Modern Uses of Cure"
William Thomas Fernie
Sachs showed, in 1860, that roots growing in contact with polished marble corrode the surface of the mineral, and Nobbe, in 1876, showed that the roots of seedlings reduce potassium permanganate, a fact which Molisch confirmed in 1887. The latter observer also proved that living root-hairs secrete substances which colour a solution of guaiacum blue, oxidise pyrogallic acid and other organic substances, and rendered it probable that they excrete some substance which inverts cane-sugar, and in some cases even small quantities of a diastatic enzyme.
"Disease in Plants"
H. Marshall Ward