Bethany, I understand your frustration.
Many women (contrary to popular belief!) don't get positive opk's but do ovulate.
Basically the opk is designed to turn positive when, say 40miu of LH is detected. For some women they ovulate at say 15 miu so therefore the positive will never occur. nb: the figures I am using are not relevant
I am one of those women. I have found for me the closest I get to positive is 2 shades lighter than the control strip - that is my positive. I find I only pick that up if I test twice daily. It is generally thought that the best time of day is at 2pm. Try not to drink much for at least 2 hours prior to testing - preferably 3 hours and hold your urine for 2-3 hours also. Test again at 8pm. Do this from about cd 10 if you have consistently long cycles. Do you chart your cycle? If so you would have an idea when you ovulate if you do from that also.
The cd 21 test is something I would do next cycle so you can confirm that your body is ovulating.
OPK's are wonderful things if they work but can be very frustrating and stress inducing if your body doesn't register the surge. Some women also have a 'short, sharp' surge that is gone before it is detected, so to speak. Don't despair that you are not ovulating until you get the blood test done. However bare in mind that the test is done 7 days after you ovulate. So, if you ovulate on day 21 then there is no point getting the blood test done until you are on cd 28. CD 21 test is a misleading name. It is called that because the medical profession base a normal womans cycles around 28 days. Many of us however don't fit into that box!
What the blood test is is a serum progesterone test. The progesterone rises after ovulation. It is generally thought that a serum progesterone level of approx 20 indicates ovulation has occured. For you I would average out your cycle length, that is look at your cycle lengths for the past 6 months and find an average length. Count back 7 days from when your period begins. So if you have an average cycle length of 33 days you would need to get your blood test done on cd 26 - give or take a day.
It is also worth keeping in mind that many women have differing luteal phases. So, it is possible that a woman ovulates on say day 13 but has a lutel phase of say 16 days, so even though her cycle is 30 days long her ovulation doesn't happen on day 16 so just simply counting back from when AF arrives and saying that is when you ovulate may not necessarily be correct. I have a 15 day luteal phase. Some women have shorter and some longer. However, generally the LP is 14-15 days. Charting your cycle will give you this information.
I wish you all the best Bethany. This is often a hard journey, but once you discover what your body is up to I am sure things will fall into place. Also for me I found Lullaby opk's the best for me, and they are cheap too, especially when you are testing twice daily.
Lots of love,
Deb