No problem, I've been going through info as I'm moving B raspberries closer, so I've been looking through publications
The issue is red rasp can be infected with many diseases that do little damage to them, but can be significant if black rasp are infected with the same diseases.
The recommendation of spacing is related to spores traveling through the air (anthracnose), and aphids (virus vectors).
Anthracnose can be significant on Black, but can be reduced/controlled with several cultural practices including dormant pruning, keeping weeds controlled and not over fertilizing. Preventative fungicides are available (chlorothalonil, captan, lime/sulfur and others), and curative fungicides are available although these are very expensive.
The other issue is aphids transmitting virus from reds to blacks. Many viruses infect red raspberries without causing symptoms, but when they infect black rasp can cause damage. Aphids moving between the two types will spread the viruses. Insecticides are available to control aphids, but I typically ignore them and don't worry too much. I probably control a lot of aphids on the reds spraying for Japanese beetles in June/July.
Keep in mind that no situation is ideal for spacing raspberries/blackberries. Cultural controls can have a significant effect on reducing disease, the biggest being keeping them pruned and weeded to increase airflow through the canes. A spray or two at the right time early in the season is effective as young green canes are particularly susceptible to infection.
Think about the disease triangle. Three sides needed for infection. First, causal organism must be present. Not going to do much about that, the diseases are all over the place.
Second is a susceptible host. Anthracnose primarily infects young tender green canes, protect them with a fungicide and no disease. Also may reduce infection rates by altering conditions when canes are susceptible.
Third is conditions favorable to disease infection and growth. Pruning to allow more air flow will reduce infection as canes may dry out before infection can occur.
Only really touched the surface on Plant Pathology 101, but it gives you an idea. Lots of links, spraying guides and cultural guides for rasp/black berries on Google
I've got to move my Black raspberries, and don't have a convenient place as far from the reds as they currently are. I plan to focus a little more on keeping them pruned/weeded better and maybe a dormant fungicide spray and living with what I get.
If one followed rigorously the recommendations for spacing and crop rotation, none of us would ever have a garden, fruit, etc. All the recommendations are best practices, but in the real world you do the best you can with the space/resources available.