GW190425 was the first event to be confidently detected based on data from a single detector. This means that the localization on the Earth's sky was likely not as good as if multiple detectors observed it. As such, no electromagnetic counterpart was observed for this merger.

Although LIGO/Virgo could not rule out that one or both components were black holes, it is unlikely, theoretically speaking, that the components were black holes because there masses are well below the expected lower limit of the mass gap between neutron stars and black holes. However, the existence of such a mass gap is uncertain (e.g., see this).

Officially, the remnant of this merger is considered to be a black hole, as the mass of the remnant is generally too high to likely be a neutron star. More technically, the merger formed a neutron star very briefly which very likely collapsed promptly into a black hole. The original analysis by LIGO/Virgo found a $>95% probability for a prompt collapse into a black hole (see section F.6 of this).

The source frame chirp mass and total mass reported by LIGO/Virgo are larger than those already known of binary neutron stars in the galaxy. If one considers many possible formation scenarios involving neutron-star binaries and black-hole neutron-star binaries, then it's most likely that this event resulted in a black hole remnant, and such models imply that if there was an electromagnetic counterpart it would have been redder and fainter than the counterpart of GW170817 meaning that electromagnetic observations may have missed the counterpart. However, high mass neutron star binaries are not generally expected to produce observable EM counterparts.

Assuming that the sky was searched for an EM counterpart with sufficient accuracy and there was no kilonova to be seen, numerical relativity simulations can also be useful for constraining the mass ratio of binary neutron star mergers.

Regarding your question about using ringdown to constrain the type of compact remnant, the ringdown signal for black holes formed in BNS mergers is at frequencies of several kHz and is not easily measurable by current LIGO/Virgo detectors. The original analysis of LIGO/Virgo attempted an unmodeled search (meaning without assuming a particular type of source) for the postmerger and found no statistically significant signal (see section F.7 of this). Future detectors, such as 3G detectors, will more easily access the kHz regime, where tidal deformability in the ringdown may be observed for neutron star remnants and may be used to determine the type of remnant.

Another, very uncertain and not commonly accepted, alternative is that the compact remnant of this merger was a strange quark star, but this depends on the very uncertain QCD models for the equations of state of different types of strange quark stars, and depends on how one converts b/w baryonic and gravitational mass for these uncertain stars. See this review for other exotic explanations.

GW190425 was the first event to be confidently detected based on data from a single detector. This means that the localization on the Earth's sky was likely not as good as if multiple detectors observed it. As such, no electromagnetic counterpart was observed for this merger.

Although LIGO/Virgo could not rule out that one or both components were black holes, it is unlikely, theoretically speaking, that the components were black holes because their masses are well below the expected lower limit of the mass gap between neutron stars and black holes. However, the existence of such a mass gap is uncertain (e.g., see this).

Officially, the remnant of this merger is considered to be a black hole, as the mass of the remnant is too high to likely be a long-lived neutron star. More technically, the merger formed a neutron star very briefly which collapsed promptly into a black hole. The original analysis by LIGO/Virgo found a >95% probability for a prompt collapse into a black hole (see section F.6 of this).

The source-frame chirp mass and total mass reported by LIGO/Virgo are larger than those already known of binary neutron stars in the galaxy. If one considers many possible formation scenarios involving neutron-star binaries and black-hole neutron-star binaries, then it's most likely that this event resulted in a black hole remnant, and such models imply that if there was an electromagnetic counterpart it would have been redder and fainter than the counterpart of GW170817 meaning that electromagnetic observations may have missed the counterpart. However, high mass neutron star binaries are not generally expected to produce observable EM counterparts.

Assuming that the sky was searched for an EM counterpart with sufficient accuracy and there was no kilonova to be seen, numerical relativity simulations can be useful for constraining the mass ratio of binary neutron star mergers.

Regarding your question about using ringdown to constrain the type of compact remnant, the ringdown signal for BNS mergers is at frequencies of several kHz and is not easily measurable by current LIGO/Virgo detectors. The original analysis of LIGO/Virgo attempted an unmodeled search (meaning without assuming a particular type of source) for the postmerger and found no statistically significant signal (see section F.7 of this). Future detectors, such as 3G detectors, will more easily access the kHz regime, where tidal deformability in the ringdown may be observed for neutron star remnants and may be used to determine the type of remnant.

Another, very uncertain and not commonly accepted, alternative is that the compact remnant of this merger was a strange quark star, but this depends on the very uncertain QCD models for the equations of state of different types of strange quark stars, and depends on how one converts b/w baryonic and gravitational mass for these uncertain stars. See this review for other exotic explanations.