According to Dictionary.com, and to believers I'm sure, to be blessed means to be "favored divinely or by a supreme being". But to be lucky simply means you won the coin flip.
Luck is largely predictable. If a blindfolded pedestrian crosses a busy highway during rush hour and manages to arrive at the other side safely he should consider himself damn lucky. However, if said pedestrian has a death wish and tries the same feat a second time the law of percentages dictates he will very likely be crushed by a passing sedan. The pedestrian isn't unlucky, necessarily, just ignorant. Now, if that same blindfolded pedestrian attempts to cross a backwoods country road at 3am and is lambasted by a cement truck, he would be damn unlucky.
Determining what distinguishes a blessing–specifically what distinguishes a blessing from sheer luck– requires more than applied mathematics. For example: Let's consider the fabricated case of Matilda. Matilda is a single mother struggling to raise three children. She works full time as a secretary for an accounting firm and somehow manages a part-time graveyard shift stocking shelves at Kmart. She struggles to put food on the table, pay her rent, pay her heating bill, buy birthday gifts for her children, etc. You know the story. Also, Matilda is a theist (believes god can intervene in earthly affairs).
One Thursday while Matilda is on a smoke break (she smokes because she is stressed) she notices a scratch-off lottery ticket adrift in the wind. She plucks it midair, scratches it with her sole quarter and identifies three matching pictures of burlap sacks imprinted with dollar signs. She's a winner!
Is Matilda lucky or blessed (or think she's blessed)to have unwittingly stumbled upon a winning lottery ticket? I think the answer depends on the size of the prize. If she wins one dollar, she's lucky, but minimally so. Same deal if she wins five dollars. But any prize of ten dollars or more buys bread and milk. Matilda feels genuinely lucky. $50? This could be the luckiest day of her life. $100? Matilda will retell the story of the lucky winning lottery ticket at water coolers and Thanksgivings for years. $200? $450? Hot shit! Matilda is one luc-kay gal! Or, is she blessed?
At what dollar amount, exactly, does Matilda's dumb luck become an honest-to-goodness "god shined on me today" blessing? We know Matilda's living/life situation. But degree matters: Is she relatively happy? Has she been simply "in a rut" these last few years, or is she completely desperate? Are the utilities in arrears? Rent? Have loan sharks ordered jack-booted thugs to feed her to a wood chipper because she defaulted on a back alley deal? Other variables are at play too: Are the kitchen cabinets or fridge bare? Is Christmas coming up? Did Matilda recently pray, "God, give me a break or I'm going to feed myself to the wood chipper?"
Most likely Matilda's answer to this dilemma will be a knee-jerk reaction; she'll know if she's lucky or blessed the instant her brain registers the dollar amount of her lottery winnings.
For the sake of argument I'll assume Matilda is: happy but not fulfilled, current on rent/utilities but living paycheck to pay check, raising three solid C students, not being stalked by hit men but might be being stalked by the weirdo mail clerk Chauncey who graciously offers the uneaten half of his tuna melt sandwich daily, etc. I'd peg the dollar amount at which a winning lottery ticket transitions from lucky to blessing at $500. If Matilda's situation were more hopeless, I'd drop my estimate as low as $50. If she were in a more desirable living situation I'd go as high as $5,000.00, or more. But on average, I believe that if Matilda happened upon a $500 winning lottery ticket she'd exclaim "Thank you Lord for the blessing."
The wild card factor in the equation is the free ticket prize. Matilda might consider herself marginally lucky for finding a lottery ticket that awards her a second chance at a cash prize, but feel bummed if said free ticket is a loser (or if the original ticket is a loser). Ultimately that scenario is a wash, if not a disappointment. But what if Matilda gets crushed by a meteorite while walking to the 711 to claim her free ticket? That would make her extremely unlucky (or un-blessed perhaps—more on that later) regardless if the next lottery ticket on the roll is a ten million dollar winner. But if the meteorite lands three feet behind her (which is either extreme luck or blessing in-and-of itself) and the cashier rips off that ten million dollar ticket and hands it to Matilda, no theist would hesitate to claim that Matilda has experienced a blessing of the highest order.
Believers in blessings need to congregate at a weekend retreat and determine a workable formula in which one can plug in the variables and deduce whether or not an incident is due to luck, or god's intervention.
Consider again if the result of the formula is a negative? What if Matilda's apartment burns down while she's scratching the film on a surprise ticket? If finding a one million dollar winning ticket and returning to an intact apartment equals a blessing, than surely finding a losing ticket and returning to a destroyed apartment equals an un-blessing. God raised Matilda's hopes by delivering a lottery ticket, but dashed Matilda's hopes when only two burlap bags imprinted with dollar signs revealed themselves. Plus, god burnt down Matilda's house. Unlucky? No. Unblessed.
Remember the science adage: Every action has an equal and opposite reaction. I think this should apply to the determination of blessings. If a positive incident is proclaimed to be a blessing, an incident "equally negative" should be proclaimed to be an un-blessing. (The same calculation should be applied to luck, albeit the dramatic takeaway is lacking.)
If Matilda were not a theist, every incident--regardless of its likelihood--would be a measure of luck. Doesn’t that simply the equation immeasurably? And didn't your 5th grade math teacher always stress the importance of simplification?