Einstein's special relativity finds that length is contracted if the observer is moving at high speeds. Now, assume there is a mass at rest in space that is below critical density of a black hole. If an observer were traveling at high speeds past this mass, the object will contract; therefore, placing the object above critical density. Are the existence of black holes relativistic or did I make a mistake?

I don't think just traveling past it at high speed would have any effect. The object would itself would have to be moving at a high speed to contract. Don't really know if this could move the density up and create a black hole. Interesting question but I doubt you'll get an answer here.

The object just contracts relative to the observer; the object itself doesnt change in length, it just appears to to the observer

and black holes arent relativistic, they're stars that have imploded on themselves which so much mass its sucks objects in and doesnt allow light to escape

black holes..

the object does change in length. frame dragging? static mass increase?

object moving contracts relative to the below critical mass object observer
wouldn't actually change in density