Blood meal contains more sodium (by weight) than potassium, phosphorus, or calcium. So, if you plan to plant particularly salt-sensitive plants (like beans, carrots, bulbing onions, and strawberries), I might be concerned. I imagine sodium would leach from the soil over time, as it rained; so, applying it earlier would probably be marginally better. I doubt if one application of blood meal would be a big problem, but I can't say.
Blood meal is said to keep some kinds of animals away, I've read. I imagine this effect, if true, fades over time.
Blood meal can potentially burn plants, if you add too much or put it directly by the plant, due to the nitrogen. So adding it earlier might be a buffer against that. The potassium in the sul-po-mag should be helpful there, too, since extra potassium seems to be a buffer against a certain amount of nitrogen toxicity. If you don't believe me, take a look at how much potassium nitrate you can apply at once versus another kind of nitrogen (e.g. calcium nitrate). However, my potassium experience is with potassium sulfate, monopotassium phosphate, wood ash, and general fertilizers, and my nitrogen experience is with general fertilizers, ammonium sulfate, urea, calcium nitrate, and monoammonium phosphate.
I've never used sul-po-mag (but I have user magnesium sulfate and potassium sulfate). Changes in pH and toxicity are the only things I'd be that concerned about with the sul-po-mag.
Toxicity shouldn't be an issue for potassium even in high amounts. They probably wouldn't have recommended it if the magnesium and sulfur would would be toxic.
According to this site (and a few others), Sul-po-mag isn't supposed to raise the soil pH. It's said to have a neutral pH. This is all assuming it doesn't react unfavorably with your soil and blood meal.
Watering it after you apply it, as you asked regarding in the comments would probably be helpful with the blood meal, to help the nitrogen mix in with the soil.
Adding both at the same time, sooner than later, and watering it seems to me to be a reasonable course of action, but I don't know everything.