Blech-Heating food on a blech on Shabbos:
Heating foods on a Blech on Shabbos retain the same restrictions as a covered flame in which case one must fulfill all six Chazarah conditions to return a pot of food onto it on Shabbos. Thus, if the area can heat the food to Yad Soledes then it is forbidden to initially place food on it. However, in a time of need, one may be lenient to return a pot to the blech even if he placed the pot down and let go of it, and is thus missing one of the conditions of Chazara. However, as already explained, it is best to place an upside down pot on the blech and then place the food on top of it.
Areas on the blech that are not directly over the fire: Have the same status as areas which are directly over the fire.
If the entire area of the blech cannot heat the food to Yad Soledes: In such a scenario it is permitted to place [even uncooked] food on it.
If parts of the blech can heat the food to Yad Soledes while other parts cannot: In this scenario there is a dispute amongst the codifiers if one may initially place food on the non-Yad Soledes areas. Practically one may be lenient when in addition to this there joins in other leniencies. [Such as it was originally on the blech but was removed without intention etc.]
 Shabbos Kehalacha Vol. 1 page 411-414; Piskeiy Teshuvos 253:28-29 and footnote 264; SSH”K 1; Igros Moshe 1:93; 4:74-32
Other Poskim: Some Poskim are lenient to permit heating all foods on a Blech. [Zera Emes; Mahram Brisk 2:76; Maharam Shick 117; See Shabbos Kehalacha 10 footnote 23 p. 412; See Piskeiy Teshuvos 253 footnote 264 in name of Degul Merivava]
 Based on 253:26. So rules the Igros Moshe and Shemiras Shabbos Kihilchasa and other Poskim today.
 This leniency is allowed being that there are many Poskim which even initially allow one to place fully cooked food on a blech, as they do not consider this similar to Halacha 26 being that here the blech was explicitly placed with intent to cover the fire, and thus according to all there is no suspicion that one may come to higher the flame.
 See Piskeiy Teshuvos 253:29
 Shabbos Kehalacha ibid based on 253:21; Igros Moshe 4:74
Background: It is permitted as it is exactly similar to a Kirah oven which has a covered fire in which we rule that one may place even uncooked food on top of it in an area which is not Yad Soledes. However, Tzaruch Iyun as the stringent opinion [mentioned in 253:26 or Halacha 4 above] rules that even if the top of the heater is not Yad Soledes, one may not place it there directly without first placing another separation between it. However, perhaps here by a blech which is not Yad Soledes there is more room to be lenient, as some allow even initially placing a cooked food on a Blech being it is not similar to a heater of back in the day. Hence certainly one should be allowed to be lenient by an area which is not Yad Soledes. Vetzaruch Iyun as perhaps the same way we are stringent on a Blech by an area that is Yad Soledes, perhaps we are also stringent by an area which is not Yad Soledes, as the same suspicion applies in both areas [according to the opinion which compares Blechs to heaters of back then]-perhaps one will come to stoke the coals! Practically Rav Farkash rules leniently in this matter and so rules Igros Moshe 4:74.
 See Piskeiy Teshuvos 253:29