The first lever filler Blackbird was introduced in 1922. I suspect that was the Blackbird Self-Filling Pen with no assigned model number and a spoon feed. Sometime soon after that (I assume) it was replaced with the BB2/60 which had a ladder feed. As the Thin Fountpen resembles the BB2/60 so closely and also has a ladder feed, it would be my guess that its dates are roughly the same.
Why did Mabie Todd produce the Thin Fountpen? It’s unlikely we’ll ever get an explanation, so we’re free to speculate. Fountain pens started thin because they copied the style of dip pens. Around 1920 they started getting noticeably thicker, as their dip pen ancestry drifted into the past and people wanted a pen that held plenty of ink in a sac inside the barrel, necessitating a thicker pen. That might not have suited everybody, and there may have been customers who asked for a traditional, thin pen. Perhaps the Blackbird Thin Fountpen was the result. They’re not common, perhaps because the demand for them turned out to be less than expected and they were dropped after a short production run.