Fake blood like any makeup, will stain certain materials, even the ones that state that they are non stain on the product. I would suggest testing the blood, on a piece of the same material as the blouse you plan to use. Most proprietary brand fake blood, has a food dye or cochineal dye based colouring agent & may also use ?Photo Flo? to increase it?s viscosity - which can be harmful to the eyes & if ingested. - Read the instructions very carefully before using.
There are loads of recipe for fake blood or Kensington Gore on the net, if you wish to make up your own, which is cheaper if you are using large quantise. However most  home made brews, use milk products & would have to be kept in the fridge, until you use it because it will deteriorate fast. The same with purchased products they also have a limited shelf life after opening.
As a side issue ?Kensington Gore? is used as a name for Theatre Blood, meaning fake, the same is used to describe the  ?put on? style of English accents used by the Pohmy  BBC announcers, on the radio & MC?s at the ?Albert Hall? years ago -that they are famous for. Because the ?Hall? is located on the site or an original ?Gore?. A triangular piece of land or green sward in London. This is a corruption of an ancient word ?Garra? meaning triangular &/or ?Gar? a spear head. Not as for the site of a historic blood bath or infamous battle, as some believe. Hence it was adopted by theatre for fake stage blood.

Just a follow up on my hunt for washable blood.  I got some good advice from a professional costumer friend of mine.  He advised that the costume should be polyester if possible, and that liquid laundry detergent with food coloring worked pretty well if you got it soaking fast.

Here is the easiest recipe I found online.  If anyone uses it before I do, let me know how it works  :)

Wash-out Blood: Washes out of almost anything even after being smeared in and left under stage lights for 30 minutes. Does not dry well at all. More expensive than scenic, but looks great. Mild irritant if prolonged exposure to skin, not at all potable.

Pick up a jug of liquid Ultra Tide. It should be light blue in color and a pretty good blood consistency. Add water or corn starch if you need to change thickness a little, but it can't change much. For a gallon of tide, a bottle of red food coloring should be added (more or less) a drop of blue food coloring might be too much so be careful. A little yellow will do some interesting things to it. For lighter colors add flour/starch to add white. I'd suggest pouring some tide into muffin tins to play with color until you think you know what you want. I have never used another brand of detergent for this, but Tide has come out of Lady Macbeth's white gown again and again.

 

Hree are a couple of links:

http://www.makeupmania.com/details/IA-0021-10-000.cfm

http://catalog.cineshoppe.com/viewProduct.cfm?item_id=484451

http://65.18.207.26/store/page162.html

 

But ugh, the descriptions are a little creepy!!! Which I guess is a good thing...