For blanket stitch, many websites recommend topstitching thread for the top (regular thread in the bobbin). Harriet Hargrave recommends 30/2 thread; I don't know if that's the same as topstitching thread or not.
I think what I have used in the past is #9 crochet cotton (much thicker than topstitching thread, comes in balls rather than spools; have found it in quilt shops and other places).
You can also double-up a thread to get a heavier effect. Try doubling up your regular thread. To do this, put two spools on the machine and thread together as if they were one thread. Some people recommend threading one strand through the right side of your upper tension disc an the other through the left side of the tension disc, otherwise treating them as one strand; I have done this too. I would not recommend trying this with #9 crochet cotton; too thick!!!
You will probably have to experiment a little to determine what effect you get from different threads and doubling up on a thread.