Typically, soil organisms become active when soil temperatures exceed 45F. Some blood meal will become immediately available - it tends to be some of the most soluble of the organic amendments/fertilizers and care should be taken to avoid it in direct contact with plant roots. It can burn.

Lee: I know that this isn't an answer to your question, but I wouldn't use blood meal at all -- it's far to strong. I would use legumes in rotation for nitrogen together with compost; if you need a quick boost I would depart from organic practice and use a little artificial nitrogen. Regards', Peter.

It can also attract animals which then dig where it was applied.

The lettuce will probably start growing with no fertilizer. Blood meal does seem a little harsh and salty for spring greens.

I wouldn't put any blood meal where you are going to plant the beets or any other root vegetable as N tends to promote lots of top growth at the expense of the root. Unless you are growing the beets for the greens of course.

Your plants' nitrogen needs are correlated to temperature, too. So when it's so cold that the blood meal won't release much nitrogen, your plants won't need much nitrogen either. When it warms up, they'll need and get more nitrogen.