I want to do some blend but I don't know what seems to be the proper way of doing it. Should it be blended during roasting or blend it after roasting?

i found the link below and it seems it is roasted already.
/espresso-blending.html

and it seems that blend requires robusta therefore it's a must to have.

Some blends are blended post roast and some pre roast. It depends on what you are trying to achieve. I do both.