As the title says. If someone places the robber in one of the outer hexes which has a settlement or city that lets them do 3/2:1 trading, does the presence of robber block them from trading?
No. From the almanac, p11:
Players with settlements and/or cities adjacent to the target terrain hex receive no resources from this hex as long as the robber is in the hex.
That's it; the robber rules say nothing about maritime trade and the maritime trade rules say nothing about the robber. In the absence of some rule telling you there's an exception, the normal rules must be assumed to apply, i.e. you can continue to trade normally.