cakes are really prone to bruising, so if it's truly blue, and doesn't smell like anything other than mushrooms,
you're good
I agree... if you haven't yet had a flush the mycellium is still strong. Because it has a hold over the whole substrate it is highly unlikely that a foreign spore would be able to germinate on your cake. When you get into dunking and going for 2nd and 3rd flushes when the mycellium weakens is when you going to want to watch for contams.
Just an added note. In my experience contams are most likely to strike at two different times.
The first being while your jars are colonizing.
I've found it helpful to have a jar that you fill with sterilized grain but do not inoculate and use it as a control. If the control jar contams you know that either your grain was not properly sterilized or you have contaminated spores whether you collect them yourself or buy syringes. (Collect them yourself!! It's easier then you think and once you do that you litteraly are totally self sufficient and have to spend very little money to keep things running)
The second being 3rd and 4th flushes once things weaken.
Not much you can do about this but toss em.
Puffin