I think I expected to answer this....
I have been racking my brain and searching the web to try and answer this one. This is a far fetched explanation...
First, the blueing with an alkali you have mentioned applies to blue corn when lye is added which develops the colour blue. I don't think an alkali was involved here.
lentils (even yellow) contain anthocyanin pigments which are water soluble. Depending on the chemical present this colour can range from pale yellow to deep blue.
The canned broth was probably loaded with metallic ions (tin, iron, zinc and the like). Metallic ions combined with anthocyanin can make dark blue colour when no coulour was apparent before the addition.
This is probably not the definitive answer but It's my best guess not having seen the product nor the lentils in question.
ref: Green Lentils, All About Beans and Legumes on the Worldwide GourmetAgris Search Results
excerpt: The formation of metallo-pigmentation and copigmentation as potential mechanisms of inking formation was investigated in peach and nectarine skin tissues. Cyanidin-3-glucoside, the most abundant anthocyanin in peaches and nectarines, formed very purple ferric complexes with an anthocyanin/iron molar ratio of two