First of a blueprint is more like a holder of assets, and as far as I know have no real impact on the rendering of an object, (not counting the cost of the actually blueprint itself), and as such shouldn’t matter for rendering, don’t take my word for this tho, since I haven tested or read anywhere to the contrary, but I would wager it doesn’t from experience.
If you do want to combine static meshes you would do that outside of unreal by merging both the meshes and the textures into single assets, Simplygon is useful for this.
Finaly this question has allot of gray in it and as such is hard to anser with a yes or no. This is because separating objects kontra keeping them as one large object is both good and bad, depending on the situation, what your developing for and what performance you target platform has, large object cost less draw calls, but instead cant be culled as efficiently. Also each different material used is in effect another draw call, so a large object w allot of materials still ain’t very good and also isn’t very good for culling so. allot of gray.
It would help to answer this better if I knew what you were developing for, mobile or console/pc?
Hope that helped clarify ^^
ps: This is a good rundown of some common performance guidelines.