How did the Crusades and/or Bubonic plague lead to an increase in trade during the European Dark Ages?
The growth of trade in the Middle Ages happened as a direct result of the Crusades and not of the Bubonic Plague.
The series of holy wars, known as the Crusades, were responsible for the growth of trade because they facilitated contact between the Christian West and the Islamic east. Crusaders returned home with exotic spices, fabrics and foodstuffs which consumers loved and wanted to have more of. Similarly, crusaders took Western items to the Islamic world and trade routes were established to enable the free movement of goods.
In contrast, the Bubonic Plague, or Black Death, which appeared in Europe in 1348 led to the temporary stoppage of international trade. In fact, it was trade which brought the Black Death to Europe and which forced the trading port of Genoa in Italy to close its doors. As the Black Death travelled around Europe, from port to port, other cities soon followed suit and only re-opened their networks when the plague had disappeared.