Before you fertilize I suggest you check your PH or did you test before planting? What is your PH? If between 4.3 and 5.2ph you can just use a 10-10-10 or 12-12-12. If between 5.3 and 6.2 use ammonium Sulfate. If over 6.2, you may have problems keeping them able to take up nitrogen, over 6.8 they should be container grown with lots of peat moss.
I have grown 4.5 acres of blueberries since 1982. My natural PH ranged from 5.6-5.9. I first tilled in sulphur dust, then planted the next year.
Your spacing of 4' in row is good, at 8' between you will need to keep the bushes well pruned on each aisle side or you will eventually find it hard to drive between them to mow.
For fertilizer, 50# is way more than what the blueberries should get this year. You want 1 oz per bush this year, in yr 2 do 2 oz/bush and up it each year until year 8 at 8 oz/bush. Then stay with 8 oz/bush/year from there on. Also, you need some epsom salt (magnesium) about 1 oz in year 5, and repeat every 5 years unless a soil test says you have a good magnesium level. Fertilizing timing, you are far enough north that you only want to fertilize in the spring, farther south, like the Jersey Pine Barrens, they split it 1/2 now, 1/2 in June. If we fertilize later in the year, we get too much late season growth which gets lots of winter injury. My Cornell Co-op extension agent, back in the 80's said I would not do as well with urea, it is so strong it burns roots. The ammonium Sulfate is spread over the whole root zone out to the drip line. I use a fertilizer spreader with a banding kit so it throws the fertilizer into the row. Anytime from now til mid May is good to fertilize. In fact even late fall after the bushes have gone dormant is another good time.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.