May one place Challahs directly on the blech in order to warm?
Rav Shlomo Zalman Aurbach writes that it is allowed to place Challahs to warm on areas of the blech which are not directly over the fire, being that this is not the regular way of baking during the weekday. However, Rav Farkash rules that one should not be lenient unless there is a great need to do so.
 Shabbos Kehalacha Vol. 1 page 414-415; Piskeiy Teshuvos 253:27
 So rules also Piskeiy Teshuvos ibid to be stringent.
Is there any problem for one to place Challahs on the Blech on Shabbos for them to become toasty?
It depends on how you plan on doing so. If the Challah is dry and you place it on top of a pot that is on the fire or Blech in order to merely warm it up and not for it to become actual toast, then it is permitted to be done. If, however, the Challah contains liquid such as condensation, or ice particles from the freezer, then it must be dried prior to heating it. Now, regarding if you may heat it directly on the Blech or electric plate, this matter is under dispute. Likewise, regarding if you may toast it to the point that the bread turns hard like toast, this too is under dispute, as explained above. Practically, my advice on the subject is to circumvent the issues and not place the bread directly on top of the electric plate but rather on top of an upside-down pot or baking pan. Likewise, one should not intentionally place the bread there for long enough time for it to become actual toast although if it happens to harden while one is trying to warm it up, it is fine.
 See Chapter 5 Halacha 4F in Q&A; Shabbos Kehalacha Vol. 1 page 414-415; Piskeiy Teshuvos 253:27