Global trade will probably only be significantly impacted if the canal stays blocked for more than 2 weeks, which is when it starts to become a reasonable option (time-wise) to go south instead and round the cape. A backlog of ships on both sides will start to be really significant after 4 days of blocking, with longer blockade implying ports increasingly further down the routes being impacted as well. Currently, the worst that probably is happening is that the local logistics sector (train, truck, etc.) is in re-booking hell to handle the suddenly delayed ships.

src: I work at a container-logistics software company, and we've handled and seen similar situations (sudden loss of a main class of transport for a few days).