The number of choices for vivarium substrate is many and diverse. Some of the more common ones are peat moss, vermiculite, and potting soil. Each of these has unique properties that are benificial to different species of tarantula. Since your question was in reference to T. blondi, from what I have learned they require high levels of humidity to thrive. I would think peatmoss or a peatmoss/vermiculite mixture is what I would suggest. Both materials have excellent water rention properties, only the vermiculite inhibits burrowing unless it is mixed with another material.
Hope this helps you Justin... and again.. Welcome..