Blod is 'blood'. This is not real blood on my windowsill, but I was really surprised when I touched the withering amaryllis flower; it shed some drops the colour of blood. Maybe the botanist who found the plant* saw this too, and named it not only after the red flower but also after this splash.
Blod and blood come from Germanic *blodam, probably originating from the Indoeuropean rood *bhlo-, perhaps meaning "to swell, gush, spurt, burst out”, from which also derive English blow, blade, bloom (via Old Norse) and flower (via Latin) and Swedish blåsa, blad and blomma.
*Update: Now I have read some more and found that the plant that was originally named amaryllis is not Hippeastrum but a plant of South African origin: Amaryllis belladonna (kapamaryllis in Swedish.