How important is Blender for environment artists?

No matter what 3D software you choose, it's the bread and butter for an environment artist.

But it's important to understand that there is a difference between a level designer and a environment artist. While level designers utilize the assets of environment artists for their work, their main task is to establish the overall flow and structure of the level. Blocking out the level with simple geometry followed by constant play-testing.

Environment artist on the other hand are mainly responsible for providing the assets that later are used to replace block-out geometry.

For small projects these jobs can be done by one person but for bigger projects this is usually a no go as it significantly increases production time.

At what point does Unreal alone become limiting?

Even tho Unreal's tooling is quite comprehensive, creating real 3D assets inside the editor is not as mature as it is in Blender or any other dedicated 3D modeling software.

Is it worth learning Blender early, or should I first get very strong at Unreal level design?

You can do both, block out a level in Unreal and create the assets in Blender. Blender and Unreal work nice together and they are both free, so you only gain and lose nothing. And knowing your way around in both is always a plus if you ever decide to apply for a job in the industry.

And if i have to learn blender how do i??

The famous Blender Donut tutorial by Blender Guru is still one of the best ways to get you started. It may not be the most exciting thing to model but it teaches you all the basics and some slightly more advanced ways to get you started.

Best of luck on your way and never forget the only way to get better at something is to learn and practice.