I am a DK with both tanka nd dps gear, My main spec is blood and while in blood with tank gear when I queue i do so as DPS. How do i switch roles so OQ knows I am a tank?

blood dks are tanks
frost and unholy are dps

if you want to WL as dps, choose one of those specs

When you are in Blood and you waitlist for a group, you will automatically be listed as a tank. If this is not happening there may be a bug.

When you are in unholy or frost, and you waitlist for a group, you will automatically be listed as dps. There is no manual way to choose what role you are listing as, it is automatically based off your spec.