Yes, you can use Narrative to design and publish your blogs for Showit 

Please keep in mind that for some of their plans you can only use the Export JPG row by row feature. 

Narrative's compatibility with Showit's plans.

Showit + Advanced Blog

Both the Narrative publish feature and Export JPGs row by row features work on this plan

Showit + Basic Blog

Only the Export JPGs row by row feature will work on this plan. (We are developing a work around to be able to publish with this plan).

Showit

Only the Export JPGs row by row feature will work on this plan.

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