For example, there are multiple objects in a scene. Is it possible to blur one of them in 3D viewport, without using the compositor?
you can do a “geometrical” blur with geometry nodes like this:
But i assume you wanna have a shader blur as in the compositor, right?
or you can use mesh to volume and volume to mesh like this:
No.
You can blur textures by mixing white noise into the mapping vector but not objects.
You can do it with the compositor and use the view-port compositor. (render preview only)
You might be able to do it with a draw handler and a shader if you’re talking blur like a privacy or nudity filter thing.
The only way to do that is by using DOF, but that basically involves some position tricks to have everything in focus but the object you’d like to blur…
Basically it’s a post processing effect, more suited for compositing than anything…
Maybe in the future render engine are going to handle NPR as good as realism, in that case real time blur might become a thing…
Interesting effects, but they don’t look blurry
Ok, thanks for the info
Unfortunately, they don’t look blurry
Yes, I hope so
You probably want to find an alternative solution until then 
If the object is the only thing in the view to be rendered, one can always tweak the Filter Size to something bigger…
But as said above, the compositor is the best way.
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Unfortunately, there are several objects in the scene and only one should be blurry
Maybe motion blur, or a glass material/ mask with some roughness?
Whats it for? A still?
It’s for an animation
have it wiggle back and forth like crazy with strong motion blur
I have tried the hack out and the object movement is too obvious. I need something like Photoshop’s or After Effects’ Gaussian Blur effect, but for 3D objects in Eevee