The description page for this rose says it is susceptible to blackspot. There is nothing about that in the references, and the only member comment that mentions blackspot indicates that this rose has good resistance.

The member ratings (which admittedly must frequently be taken with a pinch of salt) claim excellent resistance to disease, but obviously without any indication of which disease this relates to.

There is one photo (Photo Id: 288945) which appears to show some "blackspot", but based on a recent conversation with Michael Garhart (https://www.helpmefind.com/gardening/l.php?l=21.288890&tab=32) it looks to me as though that spotting may be caused by Cercospora instead. The grey centres in the spots are a trait of Cercospora, and don't appear with Diplocarpon.

So the question here is: where did the "Susceptible to blackspot" warning come from?

Edit: Found this over on houzz (in the "Guillot Generosa Roses" thread) - "I used to live in Thailand (20 years ago) and now I live in southern Mississippi. We have very high heat and humidity here too. I have 5 plants of Martine Guillot and they are some of my healthiest roses. They do not have problems with blackspot or mildew and I rarely spray."

Lineage:

"Origin and Breeding Controlled pollination. Seed parent ‘New Dawn’ x pollen parent ((‘Chaucer’ x ‘Aloha’) x (‘Iceberg’ x “unnamed seedling”))"

Ref: http://pericles.ipaustralia.gov.au/pbr_db/docs/2001265.doc