Even though we do lower the pH our Deodorant is still alkaline overall. We also use a combination of 4 different deodorant ingredients so we can use less Bicarb. 

This makes the chance of darkening a lot less but it is still possible. I wish I could say there is no chance. 

If you are concerned you could try a magnesium based deodorant like Woohoo Mellow - it will still have an alkaline pH but we haven't heard of them darkening the skin.