In my garden tests have shown that the first flowers of the season have a much lower cross pollination rate than mid season flowers (>5% vs. 20%). Unrelated to this, I also have a higher success rate (fruits developing within the bag) when I bag the early season flowers. But you have to bag the flower truss before the flowers open so I call it a bud truss. I put the bag on as soon as the little curled up bud truss is large enough to fit the bag over it, and leave it on as long as there are still open flowers in the bag or until fruit get so large in the bag that it has to come off.
Each garden's cross pollination rate is different. Carolyn Male says that she has higher cross pollination early in the season (opposite of me). There could be many factors involved in this such as the ratio of tomato flowers to wild flowers in the areas that the bees are visiting, and amount of wind (which promotes self pollination by shaking pollen loose befor the bees can get to the flowers). Although I have plenty of bees around even before it is warm enough to plant the tomatoes, I also have alot of wild flowers and few tomato flowers early in the season. By mid season the main flowers in the area are my tomatoes and peppers.
Tiny halactid sweat bees and larger bumble bees bite onto the anther cone and then vibrate their flight muscles to shake pollen loose (you can see the bite bruising on old flowers), some pollen falls onto the flower's stigma, but most falls onto the bee. The bee then scrapes it off of its body into pollen sacks on its legs. The structure of a tomato's flower has evolved to be (mostly self) pollinated in this way. Taking a stick and whacking the cage or stake sets up a vibration that does the same thing to the flower. Commercial greenhouses used to use special tomato buzzers, like electric toothbrushes, to help out before they started to bring bumble bees in to do the job better. The flowers actually release pollen starting in mid morning and ending mid afternoon, so if you are going to whack your plants keep that in mind.
This is from a write up that Carolyn Male likes to cite, especially in relation to the 5% cross rate, but note the range of values.
"Close interplanting of two tomato varieties may typically produce 2-5% NCP; however, factors such as long style length, frequent visitation of tomato flowers by bees and suitable environmental conditions may produce much higher NCP values. Various studies have reported values of 12, 15, 26, and 47% NCP values in interplanted tomatoes. The wide range of results reflects the influence of different methods and variables used in these studies; however, it is clear that NCP values can be high under the right conditions."
I too love Black Krim, although it took a little getting used to because I had never had a black tomato before, so it wasn't until my second year growing it that I fell in love with it. In addition to the earthy flavor it also has fairly large seeds and quite a bit of gel (which holds alot of the flavor) so it is quite different than say a pink beefsteak. It is usually the first to ripen in my garden, even before early varieties for some reason. I don't have much luck getting early varieties to ripen early.