Can I takeback a move if I make a mistake or if there's a mouse slip? (BlitzIn)

Yes, but only if your opponent agrees. You can request your opponent allow a takeback. They can either grant your takeback or ignore your request.

You are not obligated to grant takebacks even for obvious mouse-slips. You can refuse takebacks in all situations or allow them in some. It is entirely up to you.
To request a takeback from your opponent:
BlitzIn interface:  On the right of your board you find a button named Actions. Click this and choose 'takeback 2'  from the upcoming list.
You also use this option to accept a takeback request from your opponent.
 
Alternatively you can type 'takeback'  on the command bar under your Main Console.
To accept the offer, your opponent must also type 'takeback'
The request can be ignored by simply continuing play.
To ask to take back two moves instead of one, type 'takeback 2' 

 Note: In tournaments run in the Tomato system you can not do takebacks.

For more information read "help takeback"