Many established artists use gesso as their white.
I also use acrylic gesso (Jerry's World Famous) in place of titanium white. I haven't bought a tube of acrylic titanium in 10+ years.
Gesso is made of titanium white and i believe calcium carbonate. It is very opaque, more opaque than titanium paint. I don't know if it could completely blot out a black line in one pass, but would do better than whatever comes in second place.
I'm now painting with oils so I can't use acrylic gesso as my white. But so far have no complaints with the titanium oil I'm using.