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Black-eyed susans (Rudbeckia species) tend to be pretty disease free, so it's hard to say what the problem is without more information. If they're very young plants, or if they started flowering early, they may just be pooping out earlier in the season too. (I have 7 of these plants, and I have noticed that the one that started blooming a couple of weeks earlier than the others is getting black, dried stems now too.) I know that many places around the country have had problems with either way too little water or way too much this summer. Either of those, especially if combined with prolonged excessive heat, can cause less than ideal performance. If the stems are black, but mushy, they may be getting too much water. Rudbeckia will tolerate a wide range of soil conditions, but likes rich, well-drained soil in full sun the best. My recommendation is to cut back the plants as they die out, tossing the leaves/stems in the trash, not the compost pile, just in case there was a disease problem. Next spring be sure to add plenty of compost around your plants, then water the plants deeply each time you water and allow the soil to dry out a bit before watering again. If you have continued problems you may want to get your soil tested, but these are pretty tough little plants and another season with better weather and a little TLC should see them doing just fine. Good luck!
The great thing about gardening is that you always get a chance to start over!