That’s a tough on for sure.  There are a few questions I have for you first.  How long have these plants been planted and were they all growing at the same rate last year?  Is the one that is all black planted in an area that receives a lot of moisture- or excessively dry?  What type of soil do you have— clay or sand? The chlorotic plant suggests that the pH is high- have you done a soil test in that area lately?  If you try to pull out the black one— does it pull easily from the ground with the root ball intact? Perhaps we can help you sort this out with a little investigation… thank you for submitting the question. 



Well- then it is still a mystery (I suspect the answer lies underground) about the dead or nearly dead one.  Have your niece go out and give the dead one a serious “pull”.  If the whole plant come out with the root ball intact, you may discover that it never rooted in properly or that there are signs of root rot (cankers on stems right at the base) 

The root rot fungus could also be the cause of the chlorotic leaves.  These fungi are not necessarily ubiquitous in a planting bed and may affect one plant more than another.  Although the fungi exist in soil naturally, it also could have come from the nursery as indicated in this article: https://www.tnstate.edu/extension/documents/Root%20diseases%20of%20Hydrangea%20.pdf

As with many things in horticulture, for a confirmation on the diagnosis, you would have to send the dead plant into the diagnostic lab. https://www.canr.msu.edu/pestid/submit-samples/