You are exceedingly lucky! Very few 'Bloodgood' set seed, or they set very few and only occasionally. I could speculate endlessly on the possible reasons, but my first guess would be that there is a very fertile A.p. in the area. Look around for a green one up-wind, and not too far away, a block or two. Also, look under all the various and sundry other varieties in the vicinity, your luck may extend beyond your wildest dreams. Many special varieties set very few, or no fertile seeds. A.p. (and many others) have alternating high yield/low yield years, so do not assume that you can skip collecting seeds in a good year assuming that, "I'll get 'em next year...". Evidently, last year was a good yielding year, so don't be surprised to find no seeds this year, or seedlings next spring. To save seeds for more than one cycle, let them dry out for a couple days, put them in a sealed plastic bag (sandwich bag seal is fine), squeeze all the air out and put that in the freezer until you're ready to plant a few years down the line. You will have a smaller and smaller yeild over time, but it still works pretty good. If Maple seeds get too dry, they die, so don't over do the drying process.