Mishnah Niddah 7:3: All [blood] stains that come from Rekem [an area on the Eastern border of Israel] are pure [i.e. since the Jews there are assumed to be careful to hide their niddah stains, such stains are assumed to be from gentiles and therefore pure]. Rabbi Yehuda regards them as impure, because they [the inhabitants] are converts who are in error [i.e. and thus their blood is impure since they are Jews, and since they suspected of not being careful to hide their blood, their stains are impure for they may be from a niddah]. Those [bloodstains] that come from among the gentiles are pure. [Those that come] from among Israelites or from among the Samaritans, Rabbi Meir regards them as impure, and the Sages regard them as pure, for they are not suspected with regard to [failing to hide their] their stains [i.e. therefore any stains found can be assumed not to have come from a niddah].

כָּל הַכְּתָמִין הַבָּאִים מֵרֶקֶם, טְהוֹרִין. רַבִּי יְהוּדָה מְטַמֵּא, מִפְּנֵי שֶׁהֵם גֵּרִים וְטוֹעִין. הַבָּאִין מִבֵּין הַגּוֹיִם, טְהוֹרִין. מִבֵּין יִשְׂרָאֵל וּמִבֵּין הַכּוּתִים, רַבִּי מֵאִיר מְטַמֵּא. וַחֲכָמִים מְטַהֲרִין, מִפְּנֵי שֶׁלֹּא נֶחְשְׁדוּ עַל כִּתְמֵיהֶן:

Mishnah Niddah 7:4: All [blood] stains found anywhere are pure, except those found in rooms or in the area around a house of impurities [i.e. a house used by impure women]. A Samaritan house of impurities renders impurity by overshadowing because they bury the stillborn there. Rabbi Yehudah says: they would not bury them, but rather they would toss them out, and animals would drag them [away].

כָּל הַכְּתָמִים הַנִּמְצְאִים בְּכָל מָקוֹם, טְהוֹרִין, חוּץ מִן הַנִּמְצְאִים בַּחֲדָרִים וּבִסְבִיבוֹת בֵּית הַטֻּמְאוֹת. בֵּית הַטֻּמְאוֹת שֶׁל כּוּתִים מְטַמְּאִין בְּאֹהֶל, מִפְּנֵי שֶׁהֵם קוֹבְרִין שָׁם אֶת הַנְּפָלִים. רַבִּי יְהוּדָה אוֹמֵר, לֹא הָיוּ קוֹבְרִין אֶלָּא מַשְׁלִיכִין, וְחַיָּה גוֹרַרְתָּן: