If blue is a primary colour, why then

[BlueMooonn]

do I end up with an orange splodge when I dropped bleach on my nice blue trousers?

Blue can't be mixed from other colours.
Orange is made from red and yellow.
I get dropping bleach would remove colour but why isn't the splodge white?

A quick Google will tell you possible reasons. Iron in the water turns bleach orange. Also bleach decolourisers, so blue and black turns yellow/red, which then makes orange.

Bleach is a chemical.
Chemical reactions don't follow colour mixing rules.

And there is a difference between the way colours are made with light and the way they are made with pigments.

From a artists perspective, pure blue is a primary colour but the various shades of blue are made from mixing in other colours. So your trousers might not be primary blue.

Because bleach is a chemical, not a colour?

The blue dye in your clothes will be made up of various colours to get the shade required

I spilt bleach on a beige bath mat and a black sock last week. Both have orange stains now.

Because the bleach has reacted with the blue dye and left orange

Ah! I think this is the answer.