What’s the best way to combine these 2 parts into one with smooth transitions?
Create a surface between the outer surfaces.
If the thickness of the two object sis the same then offset the blend surface to create thickness. If the thicknss of the two objects is not the same then there are various ways to deal with the difference depending on you design intent.
What is your design intent for shape of the blending surface?
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One of the methods I would try would be a 2-rail sweep. First make the upper and lower rails from the lower outer edge of the upper object and the upper outer edge of the lower object. Join each set of edges into one closed curve each if this produces pieces instead of one curve. Snap a planar curve from the upper to the lower rail. This curve should have the shape you desire for the connecting surface including the transition shape from the initial object’s side surfaces. Do the 2-rail sweep and see if it yields something you like.
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I have tried this and ended up getting the shape I want but it’s not quite right.
I also tried making a surface on both sides another way (I forgot how) but the result was the same.
That’s exactly what it’s for, it’s a hose attachment for fume extraction but more importantly, how did you accomplish this?
Just a regular loft should do it.
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That adapter would perform better if you allow more air to go through the blend shape between both ends. It must be also smoother, in order to reduce the drag and noise, as well as to minimize the risk to collect extra dust in the tight corners and make the cleaning easier.
Here is some quick test with a similar shape. I used two ways to create the blend shape: one with a single blend surface and one with several blend surfaces using split bottom circle. The latter allows a better control over the desired shape, because it has far less control points.
Note that I used a deformable circle instead of a basic circle, because the former helps with blend surfaces and other surfaces with higher degree.
I also used blend curves instead of round fillets on the upper part, in order to achieve better looking and smoother transition to the flat surfaces. Each blend curve resembles 10 mm radius fillet as close as possible while allowing for a smooth transition.
Air duct adapter.3dm (1.2 MB)
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That seems more like a shading glitch than the actual geometry being bad.
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This design is developable and a physical part can be fabricated by bending sheet metal.
Better performance with the tradeoff of much more difficult to build if the system if fabrication using sheet metal is acceptable.
Depends on the application and size. The organic transition is better if one of the following methods will be used for the fabrication; injection moulded plastic, or metal cast, or 3d printing with any material.
Alright there are a ton of replies and a ton of solutions in the replies so here’s some insight!
It turns out that this was a shading glitch as JimCarruthers mentioned.
That being said the solutions by David Cockey, AIW, Ftzuk,Cian and not to mention the amazing tutorial video by Bobi all work so they all deserve credit where it is due.
I’ll be marking Bobi’s post as the solution since people will have an entire video to reference but I would like to thank everybody that put in the effort to help me out.
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btw this reminds me of a similiar challenge I had some time ago 
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