Hello
I am an old sewer and we used to do midi's LOL I know you don't know this!!!!! anyway !!!!!!!!!!!!
you make a piece of material either black or something matching what the colour is in the blouse maybe a bit lacy but not see through cotton would do Make a V Like a square folded in half on an angle corner to corner (like a kerchief)
ok now hem the triangle all the way around and put snaps on the right side of the triangle and put snaps on each of the 3 corners and on the inside of the blouse and just snap into place
You will have to be careful to sew on the blouse only through the inside layer of fabric or the threads will show from the outside
LOL when I was young it was improper to wear low V necks and this is how we did it
Sometimes the old ways are better
Hope this helps
An Old Sewer