I was looking at recipes for canning marinara sauce and the Ball jar's recipe notes to remove the blossom end from the tomatoes.
In trying to search for the rationale I couldn't find anything. The way I've been preparing tomatoes for canning is to skin them and trim just a bit of the stem/core (I've been growing paste varieties and they don't have much of a stem).
What difference would removing the blossom end from tomatoes being prepared for canning make?
Ultimately the only way you're going to find out is to write Ball and ask. The recipe is strange in several ways, and atypical of canned tomato sauce recipes in general. For example, most canned sauce recipes have you peel the tomatoes first before cooking them with the aromatics; filtering them after saucing them is highly unusual.
That said, I can think of two possible reasons for the instruction to remove the blossom end. One is that somehow it works better with the whole idiosyncratic make-sauce-then-sieve approach.
The other is preventing rot from getting into your sauce. Among the most popular tomatoes for sauce are romas, San Marzanos, and similar Italian "paste" tomatoes. These tomatoes are also prone to blossom end rot, where the bottom end of the tomato has black rot that may even be hidden by an intact tomato skin. So cutting off the blossom end could be a way of checking for rot.
