The short answer here is "It's complicated".

Norway as a concept is actually older than a thousand years, roughly the same age as the Denmark, both were established late in the 10th century so there wasn't really a period when the Denmark existed as a country but Norway didn't. The borders between what is Denmark, Norway and Sweden have however shifted a great deal thoughout history (generally at the expense of Denmark) which is important to keep in mind here.

Harald Bluetooth did rule over part of present day Norway (mostly the area around the Oslo fjord) and parts of (present day) western and southern Sweden. This was however considered to be Denmark at the time whereas western and northern Norway was considered to be Norway, technically Trøndelag and Hålogaland (ruled by the Earls of Lade). So yes, it can be said that Denmark ruled the south of Norway, it was however not technically the dark ages but the Viking Age 1000 years ago.

Was there trade? Yes. Absolutely. Kaupang (Norway) and Aarhus (Denmark) were both important trade centers in the Viking age. It is also worth keeping in mind that the Vikings were a seafaring people. Most people, especially in Norway, would be able sailors and owners of their own boat (for travel, fishing and trade, not longships). The trade, settlement and raiding brought a relatively high degree of mobility to the region and intermarriage between the people of Sweden, Norway and Denmark would not have been uncommon.

Northern Shores: a history of the Baltic Sea and its peoples by A.W. Palmer covers the topic relatively well.

EDIT: I'm writing this from memory as I don't have the source material on hand. I'd be greatful if /u/Mediaevumed, /u/Aerandir or /u/wee_little_puppetman could give this a quick glance and make sure I didn't foul up their respective areas of expertice too much.