Right now I can't find any really good pictures of internal blossom rot but what you show doesn't look like it to me.You cut open the fruit lengthwise and what you see is usually a large black area inside.
I've grown Opalka a lot since I was the person who was first given the seeds.
http://tatianastomatobase.com/wiki/Opalka
And of all the similar types of what some call paste tomatoes,which are usually very prone to external BER, seldom have I seen that on Opalka OR I don't think I've ever seen internal either.
But these are observations made in my zone 5 area in upstate NY.
And again, I've grown it a lot for SSE listings and also initially for seed offers I've made here and there.
But if you look at the info for internal BER
http://vegetablemdonline.ppath.corne...Tom_ComDis.htm
If you do see it it is usually seen only on early fruits and then goes away since with early growth of tomato plants the upper part,the foliage,is not in balance with the root system. Which can lead to not efficient uptake of Ca++ from the soil or most other growing mediums that folks use.
I just went back to check and found that you were growing in sandy soil and that's hard to do.Maybe in the future you can think of adding some organic matter to that sandy soil, I think that might help.
Hope that helps,
Carolyn