Praise be to Allah.

There is nothing wrong with dyeing the eyelashes with kohl or mascara or any other kind of dye, if it is free of harmful effects and the woman does not appear wearing it in front of non-mahram men. There does not seem to be any reason why the eyelashes cannot be dyed black, because the basic principle is that it is permissible, and because that is similar to using kohl, and it is more appropriate to make the colour of the eyelashes match the colour of the hair of the head. But if the dye is something that prevents water from reaching the eyelashes, then it must be removed when doing wudoo’.

It should be ascertained whether these dyes are free of harmful effects, because some of them may cause infection in the eyelid and may make the eyelashes fall out. The Prophet (blessings and peace of Allah be upon him) said: “There should be neither causing harm nor reciprocating harm.” Narrated by Ahmad (2865) and Ibn Maajah (2341); classed as saheeh by al-Albaani in Saheeh Ibn Maajah.

And Allah knows best.

(Source - Islam Q&A)