right..

right, first thing is first, im not very knowledgable with sex.

is it a good thing if the girl blushes..?

wait.. you mean generally? lol

no, whilst having sex.

if she's shy, then it sounds realistic to blush

Are you sure she was going red blushing, or going red because she was getting hot?

What Wayne said could be right.
But one thing's for sure, it's not a bad thing if the girl blushes. Quite often the girl is a bit iffy about nakedness, even in the dark. So over time couples get more comfortable with each other and it won't be a factor anymore.
xxx

I just think it's cz she's shy

It happens naturally when people have sexual activity, the girl gets hotter and many other things. :P

Oh, good, was worried i was doing something wrong. xD

thanks, +rep all round.