There’s already a lot of good replies here, but I just want to add that Roblox is really weird with materials and verts in Blender.
Here’s 3 things I always have to do once I export something out of Roblox.
Merging Split Verts
Select the object once it’s in Blender
Go into edit mode
Vert Select
Select everything
Press M to merge, merge by distance
Merging verts fixes small cracks you may see on your mesh.
Fixing the weird lighting
example,
Select the object stay in object mode
on the right panel click the green triangle(object data properties)
click Geometry Data
Clear Custom Split Normals
I made a guide on how to fix lighting if you need help here,
Separating materials into different object groups
Go into edit mode of the object
face mode
select everything
right click anything
separate > by material
I personally cannot get Roblox’s materials that are automatically exported to work properly in blender, so I clear them manually and just replace them with custom materials.
Does your object contain more than one color/material?
If so separate the material as I described above and when exporting as a LEGACY OBJ select the following,
Once you get the obj file import it into Roblox and it’ll split it into different individual meshes based on the object groups.
If your object was split correctly, it’ll look something like this when bulk importing.
Once every object group finishes importing select all of the meshes that appear in your asset manager, right click, and insert with location.
If materials or colors don’t work on your meshes after importing, you may need to clear the Texture ID field on the mesh properties.
If you have any other questions feel free to ask. I have a lot of experience in optimizing Roblox made models in Blender.