Superior thread So Fine 50wt. is a wonderful blending thread that may work for you. Make a practice quilt sandwich with your t shirt scraps and batting and backing to see how it looks. I have also used Superior Threads Bottom Line, a 60 wt. thread for quilting and it disappears into the fabric and you just see the texture.
So many thread colors to choose from and lots of information on the Superior Threads website. Good luck!
Superior 50wt So Fine! Thread is a lint-free, matte-finish, smooth 50 wt. 3 filament polyester thread.
Due to its fine nature, So Fine! is an excellent blending thread and loved by both longarm and home machine quilters.