Black cats are not more likely to produce more allergens than other colors, that's a myth. It's just that black shed hairs are more noticeable than lighter colors that blend in better, so that rumor came about! But it is the dander - not the hair - that causes allergic reaction! Some of the longer-haired cats give people with allergies less trouble, their dense undercoats mean they tend to leave less dander about.

And it varies from place to place but yes, statistically, black cats get adopted slower - in a cage setting they are harder to see and literally get overlooked, and some people have stereotypes about them lodged in their brains as well, so they can languish and become withdrawn, and then even less likely to be adopted - you get the picture!