You are correct.
However, at the very first few days, sometimes a vein will just happen to grow in a way that looks a lot like a boodring esp. if your candling light is week, or the shell is dark or thick. So your sitting there trying to guess if it is a vein or a bloodring. You don't want to toss a good egg, but you don't want to risk a bad egg spoiling the others, it is a puzzle & a judgement call. That is when if its early enough that it might be a good egg w/ a confusing vein you just mark the egg, leave it another day or 2 & re-check. I often put the mark on the egg right beside where I'm trying to figure our vein vs blood ring, so it calls my attention to that area when I recandle. (The blood ring is actually the rotting veins/blood, so it will become more brown (less red) & more fuzzy as it gets more nasty inside there). So again sometimes the only way to be _certain_ is to compare the same egg to itself a day or 2 later. If it dose not smell or weep then its fairly safe to allow it another day or 2 before you decide, and that extra day or 2 generally makes it pretty obvious if you have a live embryo in there (or not). Finally, it is just a matter of experience, the more eggs you candle the more you learn to get a read on what is going on. Opening the bad ones helps too so you can confirm what you were seeing. It is scientific, but learning what to look for is kinda an artform too.
I can blame it on 4-H and the kids this time. 
