What is the interpretation of seeing a dark road in a dream after performing Salatul-Istikhara, knowing that the dreamer intends to leave his job at a usurious bank and travel to seek other employment?

It is not permissible to work in interest-based banks. Your Istikhara prayer, if it was concerning leaving interest-based work, is misplaced, because Istikhara is performed for permissible matters, not for abandoning forbidden ones. Istikhara is prescribed for you regarding traveling abroad. If you perform Istikhara to Allah the Almighty in the prescribed manner, then proceed with what you sought guidance for. If your heart does not feel at ease with traveling, you may forgo it and search for permissible work elsewhere.