You may have overreacted the litmus paper. Such overreactions are possible when indicators are exposed to strongly basic (or strongly acidic) conditions.
The complex organic compounds that make up typical pH indicators may undergo multiple stages of acid-base reaction, which could then impart more than one color change. Perhaps the best known example is with phenolphthalein. We all know what happens when the indicator, typically added inits colorless "weakly acidic" state, is exposed to a basic solution. But:
Between strongly acidic and slightly basic conditions, the lactone form (HIn) is colorless. The doubly deprotonated (In2-) phenolate form (the anion form of phenol) gives the familiar pink color. In strongly basic solutions, phenolphthalein is converted to its In(OH)3− form, and its pink color undergoes a rather slow fading reaction1 and becomes completely colorless when pH is greater than 13.
So if we were to expose phenolphthalein to let us say, a one molar sodium hydroxide solution to get the beautiful pink color, we would instead see the pink "unexpectedly" fade away. Milk of magnesia is actually better optimized for turning phenolphtahlein pink, because of the lower pH to which magnesium hydroxide solutions are limited.
So it likely is with litmus. Adding a powerful base such as calcium oxide to an already "deprotonated" blue litmus may cause an overreaction similar to what Wikipedia reports for phenolphthalein. Here, too, using the less soluble but still sufficiently basic magnesium oxide or hydroxide should avoid the overreaction.
- Kunimoto, Ko-Ki (February 2001). "Molecular structure and vibrational spectra of phenolphthalein and its dianion". Spectrochimica Acta Part A. 57 (2): 265–271. Bibcode:2001AcSpA..57..265K. https://doi.org/10.1016/S1386-1425(00)00371-1. PMID 11206560.